I have a 1998 A4 1.8T Quattro 5 speed and have had a noise that has been bugging me for some time. With the car in neutral, if I rev the engine up to roughly 2000 RPMs or higher and let off the gas, as the RPMs come down there is a ratchet noise that sounds exactly like a ratchet wrench constantly spinning. Driving is the same thing, with a slight pressure on the gas at a constant speed the noise isn't present, but as soon as there is any tension on the engine from releasing the pedal the ratchet noise will sound. I couldn't find any rattling heat shields so I don't think thats the problem. It is quite loud/annoying echoing off of houses. I held a rod on various components and listened for the source of the ratchet noise. It seems when holding it to the CAT the noise is quite loud. I have taken the CAT off and that wasn't the problem. Could this be a valve or a turbo problem? I also have an error code on my on my throttle body, I cant remember the code but its something about throttle body HIGH, and I've read that cleaning it may solve the problem? Is this right?

 

It could be a trans issue. as far as the throttle body yes cleaning it and readapting MAY fix it.

 

I had my wastegate valve become very loose and wouldnt seal with my K03. Since it didnt seal it would vibrate and could maybe cause the noise you are speaking of. I ended up getting a K04, never repaired it.