Case Study: S4 Startup Rattle

[rocky.ca]

I have already seen the thread about the rattle and whether or not members have it. I am putting a different swing on this.

When I start my car, I get a rattle 1-2 seconds after starting my car. It is more like a clack clack clack. I have read that a lot of people attribute this to the cam adjusters locking into place. Here is the catch though. Try turning on your car, waiting about 10 seconds, or until the seat belt light goes off, and then starting your car. Seems to virtually eliminate all start up rattle. For me at least.

Is this some sort of electrical device that cycles every time the car is started? Can we reduce this horrid noise by allowing it to cycle BEFORE we start the car?

So my question to you guys:

Start your car 10 times without waiting, count how many times the car rattles. Then start your car 10 times WITH waiting until the seat belt light goes off, and count the number of rattles. I want to see if there is some kind of correlation here.

I am sure that this probably affects the higher milage cars more than the lower ones, so if your car doesn't rattle at all... lucky you, you don't need to play this game

[8cylinders]

isnt that due to the fact that oil pressure has built up and tightens the chains? mine wouldn't rattle if started 5-20 seconds after being shut off but the motors just been pulled and it def needed everything done.

[dparm]

isnt that due to the fact that oil pressure has built up and tightens the chains? mine wouldn't rattle if started 5-20 seconds after being shut off but the motors just been pulled and it def needed everything done.

The oil pump isn't running if the engine isn't running...

[8cylinders]

im a noob so dont quote me but my mechanics explanation was that oil pressure stays built up for a short while even when the car is off?.....that was his explanation for why it stops rattling directly after being started but im only going off what he told me

[Mr.Wrong]

The only correlation I've found and tested vigorously is the following.
Whenever I parked it on a sloped driveway, it would rattle on a start up 7 out of 10 times. However, if I'd park on a level surface, garage, it would rattle extremely rarely I'd say once out of ten start ups. Told my two other friends(b6 s4 avant and b7 s4 sedan) to try it out and same result - much less occurrences of start up rattle whenever parked on a leveled surface.
Thoughts?

[dparm]

im a noob so dont quote me but my mechanics explanation was that oil pressure stays built up for a short while even when the car is off?.....that was his explanation for why it stops rattling directly after being started but im only going off what he told me

Yes, there is some residual oil pressure.

If the car has been sitting for an extended period, the oil pressure will bottom out and most of the oil in the motor will drain back down into the pan. When you start the car after it sits, there is going to be a brief period (literally 2-3s) of metal-on-metal contact until the oil has been pumped to all the nooks and crannies, such as the chain/guide assembly. You'll hear that metal-on-metal, and it's why some guys here have installed the Accusump.

But if you turn the car off and then start it back up quickly, as you described, there will still be a layer of oil on most parts. That's why there is less noise.

[98champ]

When I start my car, I get a rattle 1-2 seconds after starting my car. It is more like a clack clack clack. I have read that a lot of people attribute this to the cam adjusters locking into place. Here is the catch though. Try turning on your car, waiting about 10 seconds, or until the seat belt light goes off, and then starting your car. Seems to virtually eliminate all start up rattle. For me at least.

So my question to you guys:

Start your car 10 times without waiting, count how many times the car rattles. Then start your car 10 times WITH waiting until the seat belt light goes off, and count the number of rattles. I want to see if there is some kind of correlation here.

This is how my car has been since I've owned it. I might get a second of rattle on a cold morning or if it has been sitting for a couple of days if I turn the key immediately. But if I turn the key and wait for 5+ seconds before starting I have zero rattle every time. I have no explanation as to why it happens and you can hear a quick pump run but I believe that's the fuel pump kicking on.

[rocky.ca]

I think some of that rattle may be in part due to the intake manifold flapper. Apparently it cycles itself on start up. Even on these cold days, near freezing... If i let my car sit for a little while before starting it, it will not rattle. I will also mention that it has 120k miles.

[lathdogg]

I think some of that rattle may be in part due to the intake manifold flapper. Apparently it cycles itself on start up. Even on these cold days, near freezing... If i let my car sit for a little while before starting it, it will not rattle. I will also mention that it has 120k miles.

Very good point, I think the IM flapper is mistaken for chain rattle...also I feel that cold temps play a major roll in the "noises" at startup. Lets do a study on hot vs cold climate "noises", I'd be willing to bet that heated garage cars experience fewer startup noises similar to hot climates. One thing is certain, there are ALOT of variables.