Wierd Vibration!!!

Well lets start off i have a 99 v6 mustang pretty much stock when it comes to the engine. but i just got the clutch replaced and the alternator switched and a new battery. It just started happening, when im driving gears seem fine no shifting problems or anything. while in 1st 2nd or 3rd when i get on it sometimes say like 3500-4000 rpms theres a wierd metal to metal like vibration, i had my brother drive beside me and he said it sounded like something was rubbing , but he doesnt know anything about cars but it only happens when im close to 4000 rpms, oh i forgot to say i have mac dual exhaust installed on the car for close to 5 years i tried to get under the car didnt see anything out of the ordinary. but yet again only happens around or close to 4000 rpms and its a vibrations sound.
really no smell or nothing leaking. if anyone has had this kinda problem or know what it may be , it be much appreciated.

thanks

crj900 im near centerville and lynhaven parkway. What about you? hey what school did you go to. and whqt kind of mustang do you have? i have a 99 silver mustang with the black widow kit on it.

and thanks stangfanatic for the info, ill check the hangers . but i dont know yet, we'll see

figured it out guys , gotta wait till my mac muffler gets in so i can replace one of the mufflers, seems as if a baffle broke loose or is loose inside the muffler. Pin pointed like a couple days ago, thanks for the input.