Hi, my Datsun 260z has a really loud violent rear end knock. It only happens when I get the rpm quite high (above 3000) and then let off the throttle. It sounds like it's in the diff area but I could be wrong. I've had most of the rear suspension off recently and installed new springs and drop links. I've checked u joints and drive shaft bolts are tight, so I'm thinking its a bad mount.

Anyone else experienced this?

many thanks 

The nose of the diff lifts during acceleration then drops down when throttle is let off.  The knock could be the metal on the front of the mount coming down on the cross member.  If you look closely you'll see that the gap there is pretty small.

There's actually a specification for the space between the top and bottom metal plates of the diff mount.  Might be worthwhile to remove it and measure.  You can also take a large screwdriver and pry the two plates apart while it's out to see how weak it is.  You might find that it's about to fall apart.

Picture from 1976, it's clearer.

Funnily enough, there wasn't a nut on that front insulator. Oops!

thanks for the help 

That nut is hard to see.  The strap probably got a workout.