qwerty69:...a similar intermittant popping sound, which I've never made time to investigate.
Can be caused by having the sub's gain set too high, and thus the sub output from the receiver set very low, which can cause the subwoofer's automatic on/standby switching to cycle during quieter sequences. It's one reason why you shouldn't use the auto on/standby system when running receivers' autocalibration programs, but have the sub switched to 'always on'.
One solution may be to back off on the sub's gain control, and crank up the sub level in the receiver's set-up menu a good bit to compensate, meaning the sub will always be on when the system is in use.
Or, as has been pointed out, to switch the auto on/standby to always on, and remember to turn the sub on and off when required.
The pop the OP is getting is probably a muting relay lifting - the output from the amp in the sub will be muted for a moment when the sub switches itself on, to allow things to stabilise, and then it will unmute. It's really not a problem.