jeep runs fine....but i hear a slight rattling noise.. could this be serious.

I had the same problem in my 97 Jeep Wrangler Sahara. The rattling sounded like it was coming from under the passenger front seat or somewhere around there. I looked and looked and couldn't find the source of the rattle anywhere. It turns out it was from rocks(creek gravel) had gotten wedged between the skid plate and the transmission. The skid plate is located in the middle center. Try to fit something between there to fish whatever it is out. I hope this helps. Good luck