Hi guys I'm a new member with a strange issue. I just purchased a 99 A4 1.8t, and I'm gonna give you guys as detailed a description as I can of what I'm experiencing in hope someone can diagnose it. By way of introduction I've got a ton of familiarity with this engine and Ive owned several Audis in the past.

Okay so strange "fluttery" Whistly sound from the engine bay. Seemingly worst just after starting.

-ONLY occurs when in forward gear and on throttle. The turbo spool sounds healthy underneath it, and I dont really think it's the turbo, but I cant be sure. It sounds like if you were to stand behind a room fan and whistle thru the spinning blades (remember talking thru the fan when you were kids to make your voice sound weird ). Again I cannot replicate it when the car is parked and not in gear. Chirping sound almost. What I do is go to the mall parking lot and drive along side of the building to hear it better. It's louder on the passenger side. As soon as I hit the throttle, the turbo spools, and then the sound starts... the second I lift off the throttle it stops. Never heard anything like it before. I don't understand why I cant replicate it when parked, but perhaps that in itself is telling. Also, the throttle response or acceleration seems to flutter when the noise starts.

Of course most people I ask just say its the turbo but I dont think so.......I've also been told it could be the cam chain, or the diverter valve. I'm meticulous about caring for my vehicles and I would love to diagnose this....

My sincere thanks in advance for any advice!

-Paul

 

The sound you are describing makes me want to say compressor surge, but that would only occur as you let off the throttle while under boost..you are saying the sound occurs during acceleration itself?

Do you have a boost gauge you can install? (If you don't, it's worth getting one anyways.)

Can you scan for codes? You could also pop the inlet pipe off the turbo and check for shaft play. There should very little to none.

 

It's definitely related to the turbo system since you won't build boost in neutral - the engine has to be under load. Your suspicions sound logical regarding the DV. Does the car feel down on power at all?

 

There is a boost gauge installed, but I dont know how accurate it is. On hard throttle it goes straight up to 14, and then obviously flatlines after I release the throttle or disengage the clutch. I will be checking the turbo for shaft play later today.

Yes it does feel down on power. How would I diagnose a bad DV valve? It's cheap enough to replace I suppose....

 

14 is the peak? That seems too low to be chipped and too high to be stock. There could be a boost leak in combination with the fluttering problem.

First thing I would do is check the vacuum line running to the DV. It should be connected to the front nipple on the intake manifold and run down to the lower left of the engine bay (when looking at the car from the front.)

Compressor surge occurs when the throttle plate is closed suddenly (as during shifting) and the compressed air slams into the plate with no where to go, causing the chatter. The DV is there to give that air an alternative route around the throttle plate and back into the intake.

If you still have the stock DV it should be replaced anyways if you are running at 14psi. I would recommend the Forge 007, or if you want a cheaper non-servicable option, you can use the 710n DV from an Audi TT.

Is the boost gauge also a vacuum gauge? If so, what kind of vacuum are you getting at idle? It should be in the range of 16-20inHg.

It would also still be a good idea to scan for codes, there could be something lurking in there that would help solve your problem.

 

Okay guys, bunch of updates here, and before I forget thanks so much to everyone who has lent their opinion here so far.

Here are some things I have discovered:

The previous to previous (first) owner of this car, definitely chipped it with a mild program. I have discovered that he installed a very nice cat-back Borla exhaust, as well as the Forge Diverter valve (which is working fine after I removed it, disassembled it and gave it the once-over). The Turbo is in absolutely perfect working order. There is zero play in the unit up / down or side to side. It is in fact not the stock turbo but the slightly more recent K03 Sport. The oil feed lines also look much newer than everything around it.

So, we have eliminated the DV valve and the Turbo. After much more testing I will reassert that the noise is by far the worst when the car is cold, and becomes quieter as it heats up, but never fully disappears. It ONLY occurs ON throttle, and stops the second I let off, and starts before the turbo even spools up.. it is instantaneous from the second I tip the throttle. It sounds like a rotating "chirping" I suppose under some very extraordinary circumstances a belt could make that sound but I have ruled that out.

The car is currently in the shop getting a new steering rack (this is what enabled me to get the car so cheap... the previous owner brought it to the DEALER who quoted him $3,200.00 so his asking price was $3,200 less than book value, and I got him to take a thousand less than that, knowing I could get the rack for $700 installed ) replacing some dying vacuum hoses and having a shop full of experts try and diagnose the noise. At first listen... they were completely baffled.... SHOW THEM UP AUDI FORUM!! WHAT IS IT???

 

UPDATE:

Tech cannot identify the noise. Says the turbo and all associated items are fine. Does anyone have any guess at all given the description?

 

Cam chain tensioner? Can you hear it at the very low RPMs or idling? Does it sound like it's coming from the back of the head?

 

Again, I'd like to thank everyone who has offered up their opinion on this. As of yesterday this strange noise has been identified as a very very small exhaust leak in the engine bay... specifically, a bad turbo to exhaust gasket. I never could have identified this noise as an exhaust leak.... but I was referred to a genius tech who knew it right away. He gave me a heat resistant glove and told me to cup my hand around the joint aft of the turbo and, sure enough, I could feel the puffs of air.... he wrapped it temporarily and we took the car for a spin.... no more noise. I have an appt for tomorrow to have the gasket replaced.

Thanks again everyone!!