BigWhite

I have a 98 4.6 4x4 and lately it has been making an odd grinding type of noise. Body has 280xxx miles and engine and auto trans about 85xxx. The noise can only be heard when idling, at a stop, or driving very slow. The weird thing is that it only happens every like 30 secs and only lasts a second or two. It seems to be coming from under the drivers seat somewhere. Seems to stop after driving for about 10 mins. Any ideas on what it is? Is this a serious problem or something to be concerned about? Thanks for help.