Pontiac Repair: Tapping sound, ticking time bomb, powertrain warranty

Question
I have a 2005 Pontiac Grand Prix with about 58,000 miles on it.  It has an awful tapping sound when I'm driving.  The sound is very annoying and constant.  It sounds like a ticking time bomb.  I've gotten regular oil changes the whole time I've had it.   Of course I bought it pre-owned so I don't have the powertrain warranty.  Please let me know it might not be something major.

Answer
first off i guess we need to make sure the noise is coming from the engine itself?  if so, that does not sound good, can you here the tapping sound if you give it gas while idling?  if not you may have something stuck in one of the tires, ie; rock etc.