Ok on my 1971 240z everytime I drive it it makes a werid clank noise in the rear like there is a loose U-joint or something. It only makes the noise from stop to start any thoughts of what it is. Car had been sitting for 15 yrs. I just got it goin and now makes weird rear sounds.

Check all fasteners and bushings associated with the diff (front diff mount, moustache bar), transverse link, and half-shafts. Odds are you have a bad diff mount isolator or moustach bar bushings, but it could also be a bad u-joint. tha only way to know for sure is to check them all. Look for rust around the end caps on the u-joints. If you find some, replace the joint. There a lots of threads on this topic so do a search on "clunk" or "thump".