Grinding noise when idling?

I drive an 03' Columbia with a series 60 detroit engine. Whenever I am idling it makes a brief griding noise then continues to idle. The noise occurs at random time intervals. Sometimes it is subtle and brief and othertimes it is very loud and actually causes the rpm's to slow down and then catch back up to idle speed.
Can anyone tell me what this could be? And is there an easy solution?

 

This sounds like a goose chase...? I was not aware that the oil pan plug was magnetized. In the army we used to send people for riser grease for parachutes.

 

Ok does anyone have any actual suggestions to my engine problem?

 

I was thinking about it some more today and your comment seems likely to be the case. The more I concentrate on where the sound comes from it sounds like it's coming from the transmision.

In response to Mommas Money Maker, It has almost 900k miles and the last overhaul was done around 785k miles. I will talk to my boss about it, but I can already hear the complaining he will do. Somehow i'll be blaimed for it...