A Small Push on the Accelerator Produces a Short Hiss (Sound of Sipping Tea, Coffee)

[arjun90]

A Small Press on the Accelerator (1,000 - 2,000 RPM) for a couple of seconds Produces a Short Hiss (Sound of Sipping Tea, Coffee). I've done this repeatedly for a few times, and the same sound occurs. Is this something to be concerned about. I have a couple of fault codes pertaining to the Catalytic Converter below threshold, but the O2 sensors appear fine. I have two misfires; the other four spark plugs / coils are intact.

[Brandon002]

Do you have any exhaust or vacuum leaks? Exhaust leak before the cat could cause your cat codes and a hissing sound. Vacuum leaks can as well. Basically any unmetered air entering the combustion chamber or any exhaust leaking before the cat can cause this.

[arjun90]

That's exactly what I'm trying to find. Perhaps, I should post a video on this thread to give you a better idea what I'm facing here. This has been a prevalent issue for the last several months. The sound isn't really prominent, unless you lift open the front hood, only occurs for a brief second when applying the gas pedal. Current Fault Code is that is the Catalytic Converter is Below Efficiency Threshold. I have a couple of cylinders throwing mis-fires, might need a spark plug replacement or ignition coil replacement soon, though the car still starts up without a hiccup.

Do you have any exhaust or vacuum leaks? Exhaust leak before the cat could cause your cat codes and a hissing sound. Vacuum leaks can as well. Basically any unmetered air entering the combustion chamber or any exhaust leaking before the cat can cause this.

[rollerton]

Does the noise come from directly in the middle of the dash? Like up by the windshield?
Air bubble in the heater core can make exactly that sound.

[arjun90]

The noise is not coming directly in the middle of the dash; its a very faint hiss which only occurs when I tap the gas pedal for a second. It's coming from somewhere in the engine bay area, just not sure exactly where

Does the noise come from directly in the middle of the dash? Like up by the windshield?
Air bubble in the heater core can make exactly that sound.

[Brandon002]

You could have a leak in one of the flex pipes. Could open up with just the right amount of engine movement.

[arjun90]

Funny to say, I had just gotten one of the flex pipes replaced last year in August. Could be related, more like a poor quality done job by the "former" mechanic, lol

You could have a leak in one of the flex pipes. Could open up with just the right amount of engine movement.

[Brandon002]

I would definitely have it checked. Do you have ramps to put the car on? Have someone rev the engine while you're underneath. See if the leak develops when there is load on the engine/mounts.

[arjun90]

Will do that, thanks

I would definitely have it checked. Do you have ramps to put the car on? Have someone rev the engine while you're underneath. See if the leak develops when there is load on the engine/mounts.

[arjun90]

Wanted to add on that the Sound is coming within the Engine Area

[Brandon002]

Exhaust manifold gasket? Intake manifold gasket?

You were talking in another thread about misfires. Vacuum leaks can cause both the sound and misfires.

[arjun90]

I'll post a video later today exhibiting the sound. Thanks

Exhaust manifold gasket? Intake manifold gasket?

You were talking in another thread about misfires. Vacuum leaks can cause both the sound and misfires.

[arjun90]

Here's the video of me applying the gas pedal at intervals. You can notice the sound during the first second or so of applying gas,

https://youtu.be/NXOp4K73L24

Also, here is the sound of the engine during idle (I wouldn't compare this to the sound of a Diesel Engine as it isn't that loud. The fans kicked in after idling in heavy traffic on the way back home from work)

https://youtu.be/1wmBgcDQ2ig

Currently, two cylinders exhibit misfires.

Audi A6 (C5) Avant, 2004 (3.0L, Quattro)
~143,000 Miles

Exhaust manifold gasket? Intake manifold gasket?

You were talking in another thread about misfires. Vacuum leaks can cause both the sound and misfires.

[arjun90]

I thought we can embed videos in the forum.

[Brandon002]

Do you have an intake or K&N filter on your car?

That sounds like normal intake noise. That hissing you hear is the throttle body opening and gulping in air.

If you don't have an intake or K&N than you possibly have a leak in your intake somewhere.

[arjun90]

Thank you Brandon for clarifying with me regarding the intake noise.

Not sure if the air filter I have is a K&N Brand. Ironically, I drive a lot on the Highway, during Rush Hour, City and Suburbs, and there's a lot of active road and building construction. Also, I don't recall ever replacing the Air Filter (it's been exactly one year since owning this vehicle, coincidence? )
Suppose I have a leak somewhere, would it be in one of the hoses, or are we talking more extreme?

Thanks again.

Do you have an intake or K&N filter on your car?

That sounds like normal intake noise. That hissing you hear is the throttle body opening and gulping in air.

If you don't have an intake or K&N than you possibly have a leak in your intake somewhere.

[Brandon002]

Is all your intake duct work in place? Mainly the piece that goes from the air box to the grill?

[arjun90]

I'll have to check the intake duct work, I never checked the air filter since procuring the vehicle. The Mass Air Flow Sensor doesn't give any fault code and appears spotless; perhaps the duct work prior to that has to be checked, mainly the air filter.

Do I need to continue suspecting any possible leak in the exhaust or intake manifold? Or did the sound in the video not indicate anything pertaining?

Thanks

Is all your intake duct work in place? Mainly the piece that goes from the air box to the grill?

[Brandon002]

The sound in the video is air intake noise. You'd get the same sound running an open air, cone filter. It's a completely normal sound, one that is usually drowned out by intake duct work and sound deadening.

[arjun90]

Thus the misfires on the two cylinders can be simply the indicator of fused spark plugs, right? I was under the impression that a dirty air filter can cause rough idle; I usually associated rough idle with Misfires. I should hold off with the Spark Plugs and check the Air Box / Filter first, right?

[Brandon002]

Is your car showing misfires on a scan? Or are you just assuming it's misfiring because of a rough idle?

[arjun90]

I did a scan with VCDS and was informed of two cylinder misfires. The other four cylinders appear to be fine. Recently replaced Coolant Temperature Sensor which seemed to really calm the engine down thus far, but two misfires are still registering. As per the video (recorded yesterday) the PCV Valve appears intact. There is some other noise that can be heard in the video; I've noticed this noise occurs whenever after driving in heavy-rush hour traffic, usually when the Radiator Fans kick in.

Is your car showing misfires on a scan? Or are you just assuming it's misfiring because of a rough idle?

[Brandon002]

First thing you always do with misfires is to swap the coil packs to different cylinders and see if the misfires follow them.

Also, some Audis had recalls on the coil packs. Might want to call your local dealer and give your VIN.

[arjun90]

Thanks Brandon, will definitely be putting that on my To-Do list for today. Will also be checking the integrity of the Duct System as well as the Air Filter (which I am gut confident is one of the causes, but not the primary cause; this filter was never changed during my ownership). Just ordered a K&N, will be picking it up filter later today. In case the current Air Filter is clean (which I strongly doubt), then I'll return the K&N, and immediately check the Spark Plugs & Ignition Coils

By the way, I had contacted Audi earlier this year to check to see if there were any pending recalls on my vehicle (specific to my VIN); based on the history, all recalls were addressed.

First thing you always do with misfires is to swap the coil packs to different cylinders and see if the misfires follow them.

Also, some Audis had recalls on the coil packs. Might want to call your local dealer and give your VIN.

[Brandon002]

If you do get a K&N, clean it and oil it before you install it. Go sparingly with the oil though. The factory oil is applied to heavy and will dirty up your MAF quickly.

[arjun90]

Will need to go OEM, came across too many mixed comments regarding the K&N

If you do get a K&N, clean it and oil it before you install it. Go sparingly with the oil though. The factory oil is applied to heavy and will dirty up your MAF quickly.

[Brandon002]

No need for an OEM air filter. Just get whatever your local parts store has. paper filters are paper filters.

[arjun90]

Agreed Found the Mann Air Filter for $7 (Including Tax) at Advance Auto Parts, Can't Beat That. I believe Mann is the OES for Audi Air Filters as well.

[CJKaz]

Coil packs are prone to failure even after upgrade. Numerous revisions were made. Still a nagging fault on these cars. Keep a spare, they can fail at anytime.

Check PCV system, one of many Achilles heels of VW engines. Numerous hoses, valves that can fail. Under varying vacuum levels they can make unusual noises.

Old test for exhaust leaks is to hold a gloved hand firmly over the exhaust pipes to pressurize system for a short period of time (seconds). Any leaks will make quite a racket. Might need a helper to plug both pipes and listen underneath.

[arjun90]

Here's another noise I occasionally encounter for a few seconds in the beginning when starting the engine,
I have "Occasional" Idle Noise When Starting Up a 2004 Audi A6 C5 Avant 3.0L (143,200 Miles); usually occurs within the first several seconds of startup if the car has not been running for eight or more hours. This vehicle is driven roughly 60 miles every day.

Not sure what is causing this, but I recently changed over to Mobil-1 0W-40 Synthetic Oil from Conventional Castrol GTX 5W-30, and prior to this oil change, the sound was more pronounced; now the noise still occurs, but minimally, and sometimes never. During the Winter Months, the sound was more consistent. Note however, the sound never occurs while driving; only during the first several seconds of starting up the engine, and first couple of seconds of putting the gear in reverse.

Here is a clip exhibiting the noise. Thanks

https://youtu.be/OcHPg9uwAoA