Another stereo question

I'm getting a on and off hum from my subwoofer. Wire brushed all the wire ends. changes amps move speaker wires around and wire brushed the ends. This only seems to happen when i turn up the volume. If I keep at a normal volume doesn't seem to happen. Happens with the engine running or off. The only thing I haven't done is check the battery ground to the chassis or the ohms of the sub it self. Any body else have some ideas. Head unit is clarion with rockford amps, kicker sub correct size wiring nothing fancy.