About a year ago my stock flywheel died on me. Yes it does sound horrible like their are marbles chattering around. But the main problem i had within my flywheel was when the springs went bad inside of it. For the longest time i had a horrible vibration every time i engaged and dis-engaged the clutch. I also thought i might be something internal(clutch/engine) but when i took the motor out a few months ago and pulled everything apart everything is up to spec. incuding the crank. so hopefully you don't go through what i went through with a stock flywheel.
so the sound was constant with the RPMs of the engine? Was it there when the clutch pedal was released or on the floor? Like "knock knock knock knock knock"
The best I can describe my sound is like a wooden mallet with a raspy metallic sound banging against something. It is consistent with engine RPM and only occurs when the pedal is released. It is also if the pedal is not all the way released, the sound is not as loud, it's proportional.