A longshot answer
Do you have Weatherbug installed, and have you been having rough weather where you are.(We were having high wind alerts in the Wash.D.C. area last night.)
If the answer is yes, you may be hearing the "check for bad weather alert sound from Weatherbug.
If not, then it's aliens invading your mind. try an aluminum foil hat with coat hanger antenna.
I hope this is helpful