slight clunk sound in rear
 

It's only when upshifting and starting off the line. I never had this noise until after I had to have my gears replaced and had a Auburn put in. My driveshaft is a aluminum FMS that's only 3 months old. The clunk only comes after 10-15 mins driving. Yesterday I jacked the rear up and put it on jacks and whiggled the driveshaft and spun it and there is no noise. I even ran the car on the stands and shifted gears and no noise. I also checked the upper control arm bolts and there tight,so is my exhaust hangers. The lower arms are only 2 months old and tight. When turning the wheels the driveshaft it does make a slight clunk when I first turn it and the there is a little play in the rear,but it turns freely and smooth. The gears themselves are quiet. The longer I drive the more advious the noise is. My rear brakes however do make clunks when braking in reverse.

could be a few different things. at first it sounded like a lower control arm bushing. check those. then towards the end it started sounding like a UV joint, but if the driveshaft is new you probably replaced those, or it came with new ones. is there play in the driveshaft when you turn it by hand?

No play in the driveshaft. I got a response ffom someone on Stangnet and they said it could be that the stock trans mount sucks. The mount was checked by the shop that did the trans,gears and lower control arm install. My oil pans leaks about a quart of oil a week,so possible oil has run down around the mount. The lower arm bushings are heavly greased.

yeah, it could be a tranny mount. they are cheap enough to try. also look at the condition of your motor mounts.