Yo !
I have 90 Corvette, 6-speed, 42,000 miles.
My brother owns a 90 Corvette, 6-speed, 44,000 miles. Both standard 350s.

Both of these 'Vettes started this stuff in the last year:

- only "knocks" when warm, at idle;
- if you shut it off, then turn it back on, "knock" goes away
- matters not if clutch in/out, and/or in neutral.
-sounds like it's coming from "bottom" of the car....

No answers here. Misery loves company, but we'd rather have a solution than just company !!