What you're describing are the classic symptoms of a bad speaker relay in a power amp. That is:
- Can appear in either or both channels
- Changes if the amplifier is cycled on and off
- Much more pronounced at low volume levels
- Cranking up the volume for a few seconds fixes it at lower volumes for awhile
- tapping or pounding on the unit can cause or alleviate the problem
Of course if you don't have a solid-state amp with a speaker relay . . . then that's not the problem, in which case I'd second others' suggestion of bad cable connections, or loose solder connections inside a component.
- Can appear in either or both channels
- Changes if the amplifier is cycled on and off
- Much more pronounced at low volume levels
- Cranking up the volume for a few seconds fixes it at lower volumes for awhile
- tapping or pounding on the unit can cause or alleviate the problem
Of course if you don't have a solid-state amp with a speaker relay . . . then that's not the problem, in which case I'd second others' suggestion of bad cable connections, or loose solder connections inside a component.