Popping/ Thumping Noise from Airbox
 

Hello all, for the past two weeks, I've been hearing an irregular "popping"/"thumping" noise from near the Airbox/air filter. It's not loud (you have to have the hood open to hear it noticeably) and inside the car, I can't hear anything. The car drives and runs smoothly and drives fine, but I do notice that the RPM needle at idle will stay at 750 RPMs like normal for the first part of a drive then drop to around 700 RPMs (at idle) for the rest of the drive cycle. No misfiring or cutouts or anything like that but this is just what I have observed. When I scanned my car for codes with my carista adapter a couple of days ago, it stated P2177 Bank 1 system too lean off idle and P0171 Fuel trim-Bank 1 system too lean. The check engine light has not come on. But I remember a week and a half ago, the car stated my fuel cap was loose (even though I tightened it constantly for a week) then the check engine light came on. They both went off 2 days later. All new spark plugs, coil packs, air filter along with valve cover gaskets and seals were installed during the last repair. The previous owner claimed that he bought a new MAF sensor, but was still trying to chase the lean code. Actually, the idle situation was there when I bought the car, just not the sound from the Airbox. Any ideas what this could be? The car drives really good, as if nothing is wrong and the check engine light has remained off for the past week and a half and has not returned.

Hard to guess on lean codes and odd noises w/out being there. Lean codes commonly vacuum leaks, though can be other parts issues too. One vague guess is to try putting your hand on the PCV valve when it makes the noise you mention and see if you feel some vibration in sync with the noise. If so, there's your culprit. On my 2000 A6 4.2 the airbox was way over on the passenger side, but not sure where it is on yours without a picture since mine is a W12.

As I have a 4.2, my airbox is on the passenger side as well. I tried taking it apart then reinstalling but same noise.

Did you ever find the solution to this? My car is making the same noise.

Yeah I never figured out the solution to the thumping noise, and it's been almost a year since I noticed. But it is quieter now. There are no performance problems or codes/check engine light. I believe MainelyA8 found a solution and if I remember correctly, it has something to do with the throttle body. He posted his results in a recent thread (within the past two weeks). I see that this issue is getting more and more common with D3 A8 4.2s as they rack up miles. But I do not think that this problem is a major one, as I have noticed it for over a year and has not impacted performance, mileage, or any other thing.

Oh and the lean code issues were taken care of through a new right fuel pump and filter. Car is code free.

A vacuum operated intake air flap in the air box is opened as a function of load at engine speeds over 3,000 rpm to attain the large volume of air required at full throttle. This flap enables additional air to be drawn in from the engine compartment and reduces the air velocity in the air cleaner.

Malfunctioning of this mechanism could be the noise you're hearing from the airbox as its the only thing in there that moves.

I believe that my noise was caused by the rear of the air intake being very loose where it connects to the throttle body. I tightened it down, removed the throttle body and cleaned it since I was right there, and haven't heard anything since.