Ticking, Rattle noise?

[Robdog]

When I got my belt tensioner replaced along with the belt, they did it all for free and this was like a month and a half ago. But once I got it done I noticed a ticking , rattle sound coming from under my floor boards or engine, could not really tell, you can only hear it when your between 1 and 3rd gear then it stops. Does anyone know what this sound is? I looked under the car to see if any of the plastic was rattling or to see if it was not all attached but it seems ok. Its not loud but it bugs the hell out of me. Has anyone else had this problem and if so what can I do to fix it? My car has 38,000 miles on it but runs great otherwise. I hope it is nothing serious. Also it only sounds that way once you get about 3000-3500 RPM. PLEASE HELP

Thanks
Rob