Starter grinding continuously.

I just bought a new starter because my old one was starting slower and slower. Figured I'd nip that in the bud and upgrade. Went with a high torque 4:1 starter. Installed it. Went to start it and the starter make a high pitched squealing sound continuously. When I shut down the engine, I can hear the squeal subside with the engine rpm. The flexplate looks brand new so it's not worn teeth. The starter gear looks fine, no shavings, it's just not disengaging when I release the key to the "run" position.

I was told I may need a shim? If that's the case, why would I need a shim and would I be better off getting a different starter than using a shim?

Your thoughts are greatly appreciated.

cant answer definitivly but i remember a friend of mine with an old chevelle had the same problem and shims were the only way he could make his starter work properly only his was grinding on startup

I think on a small block chevy you can shim the starter but I don't think that trick works in our motors. My last new starter did the same thing and we tried shimming it for 30 minutes before we took it back for a different one.

relay may be sticking.