Same thing for me. However, when I notice it and try to pay attention to it (or change to a better tune...) it snaps back to the usual "quiet" background sound.

Yes, the brain is a scary thing indeed. Related somewhat to this, I use to do a lot of VHF contesting (amateur radio) VHF contests have a lot of voice contacts compared to other contests, so over the course of the weekend, you could spend 24hrs trying to hear voices in the noise.

And for probably a week afterwards, you hear voices in air conditioners, refrigerators, road noise, you name it. And you always hear your call sign, which most hams recognize as readily as their given names. Makes you nuts...