Subwoofer going crazy!

This is a strange one.

On the weekend I occasionally heard a low drone, like a guy with a subwoofer driving near me. This happened a couple of times...no big deal.
I was driving with the radio turned off, just the nav screen on.

Then I came to a stop and the Sub went crazy. It slowly got louder and louder then west back to a low volume. I pulled over and shut the truck off. It slowly decreased in volume until it was finally gone.

I turned the car back on, no noise. Then I reset the audio controller (pushed and held the power button until it reset.

It was a 15-minute drive home and it didn't happen again.


Now, 30 minutes prior to this happening I had driven through a terrible rain storm. Perhaps water infiltrated something??

Any ideas would be appreciated.