Interesting ... a couple of things come to mind.
Well, on my SVT when I first got that chatter, it was the heat shield hitting the exhaust.
So, do you have any loose items in your engine that you have checked?
Second, was the thought of a maybe a light weight flywheel or upgraded clutch.
Because it sounds like Clutch chatter ... common sound on upgraded clutches.
But I have also read that could be the clutch springs may not be pushing as good as if new.
These are suggestions, and hopefully you find your issue with not much hit to your wallet. And hopefully others will suggest some other theories to help you out.