First off, its a 91 RS with a 305 and a T5. It has a very loud knok that vary directly with the RPM. I pulled the flywheel cover and it serioiusly sounds like its coming from the clutch or flywheel. The noise doesnt change when you disengage the clutch though, any ideas???

 

i'd get a stethscope and start sticking it on parts to locate the sound.

 

Check the thowout bearing. My 84 z-28 had something like happen.
I couldn't tell untill the mechanic put it on a lift and engaged it from underneath.

 

A header collector leak can sound a lot like a rod bearing or a cracked flywheel.