major drivetrain/exhaust problem

on my 94 sierra 5.7 i have dual exhaust and had to change my right side exhaust donut. i had to put on 3 fix a flanges and a split flange becuase i broke all studs. when i rev up truck in park of neutral i can hear no leaks what so ever and cannot feel a leak around gasket i changed. when i am in drive and go down the road and put a little throttle in i can hear a leak. i always thought it was the donut but it doesnt make noise when i am in park. anyone have any idea to why it would make that noise going down road but not in park??

Could be that it only leaks under a load.
I would check out the manifolds, I found my manifold was cracked only after it had over time cracked all the way through, though I had been hearing the tick under load for some time.
Also have heard that the manifolds on these trucks were problematic.
Mine was cracked between the 3rd and 4th cylinder driver side.
Good luck.