rattling type noise in the pully areas on 2003 Ford Mustang GT

My 03 Mustang GT 5Sp has developed a rattling type noise from the underdrive pulley areas or at least I think it's coming from that area, sounds like it's on the Passenger side. Please help with what this could possibly be? Also any creditable mustang shops in san antonio, TX? Thanks for your help in advance.

Asked by for the 2003 Ford Mustang GT
Go to your local Napa auto parts and ask them to listen to it, It might be a worn belt tensioner or might just need some belt grip on the serpentine belt, but have the engine turned off when spraying belt grip
Valleyautotech, thanks for the information. Something I omitted in my question is that the noise is not there all the time, mostly first thing in the morning or if it has sit for a while. It appeared when it's cold and or when it rains alot and I have to go through a puddle of water. Once the engine warms up it appears to go away.