Yes I have run into this issue now a few times. It has happened to my Droid Bionic loaded with a custom rom and a stock rom. Now it just happened with my stock, non rooted, 2 week old Droid Razr Maxx.
The sound comes from the the call speaker and is a continuous ringing. It normally happens to me at night when I am sleeping and the phone is charging, but I have read about it happening to a range of Moto phones regardless of time of day and batt charge.
I'm not too worried about it because a reboot takes care of the issue, but I would still like to see what is making them do it.