> It's your differential mount. It has a
> rubber vibration dampner that
> degrades with miles & years and allow
> the unit to torque and the hard parts
> bottom out with a thunk. Part is
> about $50 from someone like Motorsport Auto
> in California. This is a problem all Z's
> eventually encounter. Mine did it for about
> 3-4 years before I finally got tired of it
> and put the replacement part in, No more
> clunk.
> Phantom.
This is one of the things it could be, to double check to see if it is your rear diff mount, take the car out, bring it up to about 4k in second gear, punch it to 5k, then let off the gas but not the clutch. If the CLUNK comes from the rear axle area, the above is probably true. If it makes more of a metallic CLINK (hehe clink clunk) from under you, it may be the joints in your drive shaft. They are easy to double check. Jack up the car, Put the tranny in gear, crawl under her, grab the drive shaft and twist. If it moves (it will jiggle a little but not much) then they are toast. I think the 76 had replaceable joints but I am not sure (I got a 78). Last but not least if it clunks when shifting check the clutch adjustment, it may be out of wak. Or the clutch just may need replacement..
Hope this helps, Let us know!!!