I was driving through a car park once and suddenly got a really loud but nice 50Hz out of the sub

Same thing happened to me driving behind a plaza. It scared the shiznittle bam snip snap snapy out of me. I reversed back by this caged communications box, and it did it again.

Probably is far fetched to be the problem in this case.
Just letting bryan know he's not the only one who's encountered this.