1964 buick wildcat motor mounts?

[rglenn]

just put my rebuilt engine in.everything runs real good,good compression revs smooth but at certain rpms and downshifting it rattles real bad.thought it was tranny and t/q converter but was wrong,then tried accesory train and still nothing.i had the engine perpetually balanced so doubt it is that.could it be bad motor mounts?any help is appreciated:Dou:

[wildcat]

i had the same problem and it turned out to be the exaust manifold valve shaft anti rattle spring...i live in ca and it does not get too cold so i welded it in the open position and no more rattle . hope this helps todd