Occasional turn off pop?

I've gotten an occasional turn-off "pop" which occurred after I added a secondary front amp (Crossfire VR302). Amp in question that seems to be causing the sound is a Crossfire VR402 and the sound wasn't present when it was run by itself.

At that time, I was running a less than ideal ground for the VR302 and needed a dedicated power cable for these two amps. After adding a better ground/power cables, the sound seemingly stopped.

On occasion I've been hearing it again once in awhile. It's minor and doesn't seem to be taxing the drivers, and it is audible with volume down when it's occurring.

Due to the amp locations, they can't share a common ground point, but both are run through a large eyelet and connected to seat track bolts that run right into the trans tunnel.

Could this be a sign of early failure? Do they use a relay in soft-start circuits? Could the relay be failing?

There's no performance issues and no unwanted noise with the audio (that I can hear) and I'm puzzled why this would be occurring intermittently?
 
I'll try and get it to happen more often and report back.

I can say it's occurred with volume at zero as I always turn volume down before shutting HU off to avoid having it too loud by accident.

Haven't been paying attention to to source (usually have CD in, but sometimes USB stick) so I'll make note. It's occurred across two different Pioneer HU, that were about 4 years apart in age.