Mechanic gave a listen to the engine and has a thought.
Happens warm or cold above a particular RPM (around 1700 right now) Sound is not at the ends of the engine Middle/center deep sound (listened from bottom)
He thinks it is definitely a connecting rod.
Ugh...
1968 Firebird 400 Convertible Original: Engine, Intake manifold, Heads, Body, Stereo, 3.08. THM400. Engine rebuilt by Ken Keefer early 2012. New: Holley 4bbl 670 HP, hidden iPod setup, RobbMC mini starter, A/C, front disc brakes