over a year ago I had the same issue, it disappeared after some months and this morning it was there again...

turning on the radio makes the subwoofer "plop" and after that it gives a constant volume, constant, low, frequency humm

turning the volume down ultimately will end the humm with a "plop" before that the volume of the humm does not change.

the humm is not affected by the revs of the engine.

MY2000 DSE with HK/Alpine highline sound system

If it's humming, then teach it the bloody words!!!

Seriously though...
Could be a dirty ground or a bad connection on the subwoofer signal wires.

The other thing it could be (seems to bring back a memory from when I was working on my sound system a few years ago... that one part of the subwoofer amplifier had failed so one of the sub cones was working, the other half of the amp had failed and was causing a hum/buzzing noise.

This was on the latest subwoofer enclosure - with 2 subwoofer cones, and a massive amplifier bolted to the back of the speakers (actually INSIDE the subwoofer enclosure). If you have the version with the 2 subwoofer cones and the black-box amp externally to the subwoofer, then I suppose it could still be the amp, but I would go checking the grounds and signal wires first.