slight rattling under the car

Hmm rock...Well, I dunno but pinging may occur under load more readily...reason I know tis not pinging is cause it doesn't occur under 4th gear (VIBRATION is differenct and I have a prof tune to BOOT, lol). I am running 440HP nitro'd and it rocks...no pinging..but WTH is that annoying rattle..always something especially when you are over th HP of a vette ...

and a vette these days is one badass car man.

I had a similar problem--turned out to be the heat shield on the aftermarket Bassani catalytic converter.
The way we found it was to put it up on a lift & gently tap the likely undercarriage areas w/a rubber mallet.
A simple tack weld or two fixed it up.