help with a noise from the back of the car.

d55dave said

Does this happen in both a straight line and corner? Tight clutches is a sure grip can be grabby around a corner if the friction modifier additive isn't used. Also, broken spider gears in the differential can make all kinds of funky noises. Have you been doing one wheel burnouts or spinning off gravel onto pavement? Also could be a tailpipe banging on so something or a loose rear u joint.

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^^^x2
Also check emergency brake cable...clips missing and half-arse rig job can give you rhythmic thumping. Don't know about the "kinda rolls funny in 1st". I had a real well respected mopar mechanic 25 yrs ago look at my 8 3/4 rear end to find a "thumping" noise that was bothersome. Couldn't find anything wrong. Years later it was the exhaust hanger slightly loose allowing the improperly rigged emergency brake cable to create quite a symphony! Cheap fix. Hope yours is too. Let us know what you find!