Help from the rear end experts

I had used Auburn posi unit put in my 69 firebird and it is making a clunking sound when I take off in 1 st and reverse. I don't notice any noise when I'm driving it down the road. It is only when I am barely moving with and with tention on the wheels. It will even do it with the clutch in. For example I notice when I and pulling strait forward or in reverse in and out of garage. The posi unit came out of a 1970 olds. A new install kit was used and the mechanic used synthetic oil with the limited slip additive. Any advice what it could be?

Thanks

Use conventional gear oil. Auburn does not recommend using synthetic fluids and I have hear od other people having issues with synthetics in these diffs. I have an Auburn in my 2002 Mustang and it is quiet as a mouse with conventional (plus the LSD additive of course).