RE: OS 55AX Noisy
The LA-series of engines do not have crankshafts supported by ball bearings. The AX-series of engines, do. You cannot get noise from the same source. The crank of an AX-series engine should not move around. If it does, then the bearings could be bad. Usually, though, in mid-to-low throttle settings, you get additional exhaust noise caused by unburned fuel exiting the engine and burning in the exhaust port and/or muffler. It will vary, depending upon the exhaust system, fuel used, and propeller. It's common on larger engines, but not unheard of with engines as small as the .55.