I replaced the flexplate on my 78 GMC pickup because the old one was cracked from the transmission coming loose from the engine. After replacing it I noticed a noise upon cranking it that sounds like its not meshing right with the starter, sort of a gear grinding/whining but not for the full rotation only for about an 8th of the rotation of the crank. Idles fine with no noise, but when I put it in reverse, only in reverse does it do this, it makes faint noise like the flexplate is hitting something. It does this until it fully shifts into gear then it goes away. Doesn't make the noise going into drive. Motor is a 1 piece rear main small block, flexplate is a 168 tooth for a TH350 transmission. Shimming the starter seemed to help some but not much. Sorry for the long email, but I want to figure out what this noise could be.

heck, try another starter, that one may just be somewhat off. Especially if it's rebuilt, on those old things, I like finding brand new ones cause the old ones are often through the washing machine after being rebuilt many times