The pushrods in my SS60 resonate at about 1/2 throttle with a ST.75 in it. Sounds like the plane is coming apart. Been doing it for 8 years with this engine. I use steel rod in tubs. It still does it even with nyrod spacers installed every 3" on the steel rod. Look for loosness in pushrods, formers, hinge points, glued joints and such. As for the the engine crank rattle, most bushing engines I've heard and had did this. It goes away when you throttle up and the crank has a load on it. Not an issue and just my opinion. Could be a combination of both.
Edwin