Engine ? noise

[GMCMAX]

I'm getting a noise I think is from the engine that is making me scratch my head, hoping somebody might have some ideas to try other than a teardown...
Here's the rundown; 350 4spd, I had 3.08 rear gears and at highway speed ran about 1900 RPM. Put 4.10 gears in it and now run close to 3000 RPM down the road.
The engine runs fine like it always has, but above 2500 RPM going down the road I'm getting a tap or rattle from the front. If I'm on or off the throttle it isn't there, but just rolling along it is. Can't get it to do it in the driveway sitting still either.
The noise is heavier than a lifter, but less than maybe a rod. Doesn't do it at lower engine speeds. Oil is clean, oil pressure where it always has been, have run it up to 4000 going up throught he gears and nothing, it will rattle in 3rd gear above 2500 rpm, but not on or off throttle. Backed the timing from 12 to 8 degrees to see if it was pinging, no sign of vacuum leak either.
Has me stumped.... wristpin maybe, bad input bearing in the trans?
Thanks,
Max