Unusual Sound in Speaker System???

Didn't think I would be writing in with a problem...but am stumped and maybe SOMEone out there might have had same.
A "heartbeat"-like two tone sound is coming thru speakers, only after this fine little 1er
[2011 128i MSport AT] has been driven for some miles. It continues after engine shut off and fob out. BUT goes away after locked and overnight--only to come back after some miles. Unplugging my aftermarket Bluetooth does not relieve issue. I think I may have the only personal heart monitor app in the world on this car--sounds just like an electronic heart beat LOL. I think its in radio system or a computer trying to talk to the car. Great dealer service here BTW, has heard it, and it goes in for a search--but wondered if anyone else has had this?? Love to get it solved w/o dealer, as they have to car long enough to hear it again. Can't fix what my tech can't hear!! Thanks for ANY insights you might have.