Trunk speakers thumping...

[Reasoned1]

Went to get the family some KFC for lunch and heard some strange thumping from the rear end. Stopped the car, got out and looked around, and didn't see anything wrong. Drove some more, and the ryhthmic thump immediately returned at a frequency higher than the rpm of the wheels. Threw caution to the wind and went to get our extra-crispy chicken. The sound didn't go away but, over 40 mph, it dropped in intensity. Got back home, opened the trunk, and heard a faint sound coming from the speakers (which were off). I disconnected them and, sure enough, both speakers were doing it. I assume some sort of induced signal from the drive train. The speakers seem to work fine, otherwise. Any suggestions or similar experiences out there?

[holyc0w]

I've heard weird stuff happens when the battery is on the way out. How old is the battery?

[Reasoned1]

God knows... no more than 135k miles or 13 years (this is my 2005 330xi DD). That's an interesting idea, but the battery seems fine in every way, and it's hard for me to imagine how an intruding, induced signal (possibly through broken insulation?) would be cured by a good battery, but I certainly can't claim to fathom it. Incidentally, we had a pretty good rain storm yesterday, and I have complained of an SEL after snowstorms this winter (it always clears after a couple days)... I also started using a pressure washer recently (albeit carefully). Luckily, I prefer to listen to the engine over the stereo anyway.

[Reasoned1]

Well, as always, the SEL cleared after a couple days and, sure enough, there's no longer any issue with the subwoofers. I'd say the battery must be fine (or at least not the cause) and that there must be an exposed connection somewhere.

Whoops--I spoke too soon. The SEL is still gone (noore rain), but the speaker issue is back.