So I'm deciding on buying a 300zx. Everything seems fine except for a knocking in the differential. He says that he replaced the original differential with an lsd from another 300zx. The car he's selling is 2+2 and he got the replacement differential from a 2 seater coupe. The knocking happens while shifting. What I'm asking is, what might be the cause of the knocking, should I be worried about it, and is it fixable?

 

I highly doubt it's the actual diff carrier inside the pumpkin that's "knocking". The usual "clunk" when shifting and going from reverse to forward is a dying cv / u-joint or toasted diff bushing. If it truly is the diff carrier with a continuous knock, I wouldn't drive it.

 

I was thinking that it might be the diff bushings. I'm checking it out tomorrow, but Im going to jack the car up and try to move the diff. Gonna check the bushings too. :/