valve train trouble

Thinking i may have a valve not seating or a slightly bent valve, crank the car and it seems to sound fine, rev the motor a couple times and as the rpms fall it sounds like a bad exhaust leak out the pass side header. While the engine is revved at 4-5000 rpm it doesnt skip a beat, only when ya back off the throttle and the rpms fall. It is a 602 crate motor, all cylinders on pass side has 150 compression. Could a weak valve spring cause it? Seems like if it were a valve spring it would do it while revving instead of after letting off the gas? Any help is appreciated

Check for a broken header.

If you don't know how old the valve springs are probably best to replace them. Also get a on head pressure tester so you know what you are starting with and can check to see if they are going soft.

It might be as simple as a header as RCJ stated. Had one that was burned between the two center exhaust cylinders.

#4 valve spring was weak, not enough seat pressure to close the valve correctly at low rpm, bad thing is, the valve springs was new