'01 passenger side Humming

ok so i recently bought a 5 speed sxt. seemed to be well maintained. After i started driving it around it was good for like a day and then i heard this like boing or bong noise. well it developed into a full bor hum today, only when i'm parked. doesn't do it allways and generally only when i'm not moving. The car will just hum but it sounds like nothing i've heard b4. any clue what it might be? the car overheated the other day cause they never put any coolent in it b4 i bought it it started chugging and stuff i think i heard the hum also then i got more coolent for it and alls well i still get the periodic hum from what seems like the passenger side. AC seems to still be verry cold aside from when the car was over heating then it was warm. i let it sit and idle after filling the coolent resevior (sp) and it seemes that it just needed cooliant for the overheating. it's got 76000 miles on it. what could the bong/humming noise be?