This problem is similar to this post, http://forums.corvetteforum.com/c5-t...n-revving.html but mine just started this morning. I found a You Tube video that sounds very close to the "rattle" I am hearing. Any ideas?
Note I had a squeaky belt, applied belt dressing last night, no more squeak, now this starts today. I don't think it's coming from tensioners or pulleys, sounds more from driver side of engine.

 

Sounds like a wrist pin rattle.

 

Come to think of it I did add a bottle of Techron to the tank also. Any chance it loosened up some carbon deposit, and causing a rattle till it burns away? Just seems funny it all of sudden happened today. I do regular driving about 3 times a week on highway, never heard this before...puzzling...

 

Problem Solved! : Well I believe I found the root of the rattle noise. I have never changed my belts, although they didn't look bad, and with only 58,000 on the clock. I did have that nasty squeak and thought it might be a idler or tensioner going bad, so I just hit the pulleys with some belt dressing. BAD IDEA, apparently that makes the belts really tacky which did stop the squeak, but causes another problem. The alternator has a decoupling pulley on it (at least on the A4's) and when reving it rattles like a shaking a spray can. When you don't have a load on the engine, it's barely noticable, if at all. I am replacing the belts with gatorbacks tonight,(found a local autoparts that has them in stock), but I may have to replace the alternator due to a spun bearing on this pulley. You can replace only the pulley, but for basically $40 more you could have all new and since it's now 12 years old, figured why not. The video I found on you tube may not be the same noise as mine, but it's real close. Anyway hope this may help others with trying to pinpoint the rattle noise, since when it does rattle loudly, it transfer the sound up through the engine, almost making sound like an internal problem.