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