I have a 68 Coupe with a 71 351C paired with a 71 C4 transmission. Recently I changed out the transmission filter and gasket as part of regular maintenance. The fluid was still pinkish/purple as it should be and there wasn't any metal flake in the pan. All looked good. When I put everything back together, the transmission started making a grinding noise when putting it on park. It will HO through all of the gears perfectly, but grind as you put it back in park.

Anyone have any ideas on what the problem is?

 

i have`nt been inside a c4 in 30 years .. but i believe one or two of the bolts that hold the filter on are a tiny bit longer than the rest and are meant to go into a specific spot .. if you put a long bolt in a short bolt location it will rub against a spinning part .. but i could be mistaken .. ask a mechanic ..