First of all here is the basic car info. Its an 03 a4 1.8t 5 speed manual.

So, I just picked up this audi about a week ago. It had this problem when I bought it. Basically the car runs fairly quietly until you turn the a/c on then you hear a loud rattling noise which can be heard in the youtube videos I post below. The interesting thing about it is, it only happens loudly when you turn the a/c on. Also, when I press the clutch down all the way to the floor it also stops the noise (if the a/c is on). This only happens at idle speed if you rev the engine a little bit the noise also stops. There is also a very slight grinding sound that maybe difficult to hear in the 3rd video which happens sometimes at idle or if you rev it high then when its decelerating you can hear it. The first two videos start off with the a/c off then I turn on rev the engine you can hear it go in and out and maybe in one of them I press the clutch and the noise stops.

The noise appears to be coming from the back of the engine, if I go listen down below the car by the wheel well its pretty loud. One other random bit is when I turn on the a/c and the noise starts on the front of the engine you can notice a lot more vibration in the serpentine belt as seen by tensioner rattling a lot when you rev it up noise stops and tensioner doesn't rattle but, I don't think its the tensioner making the noise because like I said its coming from the back.

Video 1:
Engine top low rpm a/c on off - YouTube

Video 2:
video 3:

 

Update, I have replaced the tensioner as suspected that was not the problem. Current theory is that it is oil pressure related. This seems to be a common problem based on my googling and looking at different forums but nobody ever has the answer or there is no consistent advice.

 

Take a look at the alternator and see if it has a decoupler. If it does it very likely could be that. It basically is a clutch on the alternator pulley. It sounds like the serpentine belt is slapping. Look at the serp belt running with the AC off and watch the belt when you turn it on. If it starts jumping alot then remove the serp belt and grab the alternator pulley and see if the clutching is working. Most likely its frozen up and will need to be replaced. Hope that helps. I have seen this alot on diesel engines. Replaced 2 in the last month.

 

thanks, I'll definitely do a complete inspection of the alternator decoupler pulley but, I have isolated the sound to the back of the engine and am suspecting the cam chain tensioner. The slapping noise seems to be coming from the cam chain tensioner and not anywhere near the serp belt. Also, the grinding noise is very similar to other cam chain tensioner failures, that combined with the location of the noise is leading me to abandon serp belt related theories. Unless you think I'm really way off then feel free to say so.