So, first of all I realize this is a tech question, but for some reason I was not able to "Post a New Thread" in the subforum. If I'm missing something, let me know, but the link was just not there, but it obviously is when I'm in the normal B5 A4 forum.
Second of all, Hi. It's been a while since I've posted anything.
I've searched around and just can't quite find anything that matches the same symptoms I have. I had a few ideas before digging into it, as well as a few new ideas after searching the forums, but nothing guaranteed.
Sorry if this is a long read, but I want to add as much info as possible.
Problem:
Strange noise from the engine bay. Almost sounded at first as if someone next to me had a modified exhaust when I first heard it (windows up). The sound almost sounds to me if something is turning on and turning off (it will make a sound as if it is revving up, and then make another sound as if it is revving down). I really can't describe the sound any better than that. It does continue to make this sound on and off while driving, but only when accelerating or maintaining speed. So far I have not been able to replicate it on decel or at idle. I also can't get a good video recording of the sound as it just isn't loud enough for the mic to tell a difference between the sound and my own engine noise. This sound is also associated with a very slight change in power delivery. If the rev up sound is heard, the car seems to gain a little bit of power, while when the rev down sound is heard, there is a slight loss in power. Almost as if something is catching and releasing.
Ideas:
Based on the above, I initially thought it might be the AC compressor trying to turn on and off. This would describe the power gain/loss, but the sound has never been heard before when turning on/off the AC. Just to be sure, I disconnected the wire to the AC compressor to see if it would stop the noise, but it did not.
Based on searching, I'm also now wondering if it might be a relay (like the SAI) turning on and off? I have not yet checked the relay, but.... As far as other troubleshooting, I've also checked all the fuses and came up with nothing.
Other info that may be relevant:
This all began after a timing belt change.
All belts, tensioners, etc. were changed. Water pump was changed. Temp sensor was changed. The heater hoses on the backside of the engine were also changed because of a leak.
I also switched to a lightweight crankshaft pulley (Maybe the problem always existed and with the lighter weight, now the noise and power change is just noticeable???)
About a week ago, the car went into the shop needing a new catalytic converter. This should be irrelevant but???? ***EDIT***The revs seem to stay up higher for longer in between shifts, as well. Kinda chalked this on up to the lightweight pulley, too. But maybe its relevant.
I'm just stumped. Maybe a fresh set of eyes and brains on this will come up with something I haven't thought of. I'm also still relatively new to working on this car coming from the Nissan/Infiniti side of the world and don't know as much as others on here. Anything helps, guys! Thanks ahead of time.
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Originally Posted by bmos
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