Terrible clunking noise when shifting

[Jerebus67]

I just bought a 69 RS SS 350 camaro with a M20. It is the original tranny and rearend but within the last couple of years has been totally gone through and rebuilt. There is a terrible clunking noise when shifting in 1st, 2nd and sometimes 3rd gear. (ok in 4th)
One garage said they just thought it was body flex but there is no way this is body flex. Any ideas on what it could be? Everything is tight underneath. Everything is brand new underneath.

What other info can i supply to help?

[rwhite692]

Boy... it could be so many things. What work has been done recently? You mention "everything underneath is new/tight"...

It's intersting that the shop thought the sound was coming from the body, and not from the driveline...I know you have said, "everything is tight", but I would start checking for loose fasteners. Just for starters:

Did you check the condition of the u-joints? If they were recently replaced, did you check that the u-joint retainer bolts at the yoke @rearend are tight (be careful not to over-tighten them!)

Did you check to make sure your leaf spring-to-axle u-bolts are tight, etc?

It's really hard to say where to concentrate your efforts, without more specific information...

[Payton King]

Although the bolts may be tight the crossmember could be loose or the trans mount could be broken

[Serpa69]

It sounds like you have a broken input shaft gear. The clunking you here is the teeth that are missing from the input shaft. The exact same thing happened to my transmission.