My M20B28 started making loud high pitch noise. The pitch changed with RPM and seem to reduce under load. I thought it might be a loose belt or a shot bearing. Noise was located near the back of the engine.
After a an hour or so of poking around today it turned out the intake manifold gasket was leaking on cylinder 6.
I’m pretty sure it must have been a materials failure as I’ve not put that many miles on since the rebuild. Need to order new gaskets, going to do all of them to be sure. Some non-setting gasket sealer has shut it up as a temporary solution 🙂