Hey all. I just bought my first Audi this past wednesday. It's a 1991 Audi 80 Quattro with the inline-5 and a 5-speed manual transmission. It's very clean and the list of known problems is quite short for a 20 year old $1200 car. In fact, the only things I know it needs after having a tuneup done on it are it needs two CV axle boots, a passenger side lower ball joint, a passenger side headlight bracket, and two fog light assemblies. Also, one of the wheels is very slightly bent. Can this be repaired or will I need to buy a new set of wheels? I would like to find either the same wheels or ones very similar. I really like the white-on-white look on this car. Those are the only known problems this car has.

However, there are a couple things bothering me about it that I wanted to mention in hopes that someone here would be able to diagnose it. When driving, whenever I shift normally or let off the gas all the way, there is a slight "thunk" sound which I can feet in the chassis, but not in the shifter or clutch pedal. It shifts into gear just fine otherwise and there is no whining or grinding sound. It is primarily when I let off the clutch but before I fully press back on the gas.

If I'm careful to be particularly smooth letting off the clutch along with pre-revving my upshifts or rev-matching my downshifts, it doesn't do it. But if I shift normally or shift quickly, it does it. The quicker I let off the clutch, the more severe the thunk. I'm hoping it's nothing internal within the transmission. I can't think of anything within there that could cause this, except possibly a bent or warped shift fork. Other thoughts are a potentially bad mount, mount bushing, u-joint, or hell, maybe even a slave cylinder.

Also, when I have my A/C on, the belt squeals under high torque load and I can feel it snagging a little intermittently, like it's meeting resistance. The engine doesn't run as smoothly and it feels less powerful, but as soon as I turn the A/C off the engine revs smoothly and feels considerably more powerful. Could this be due to a bad pulley or perhaps a compressor clutch? In the meantime, I'm just riding with the windows down and the sunroof open, haha.

Recently it's been a little more difficult to start and sometimes, when first cranked up and for a few minutes afterwards, there is a tapping/knocking noise coming from what sounds like the head, but it's really hard to pinpoint. It increases with RPMs, but goes away once the engine is warmed up. I thought it might be a noisy lifter and added some Lucas engine additive to see if it would help, which is doesn't seem to have. It's still making the noise just like before. It doesn't sound deep or loud enough to be a rod, piston, or crank and it's too regular and rhythmic to be a flywheel. Any ideas?

Also, how in the word do I change the air filter on this thing? It's buried!