Just picked up a good used HRX2172HXA, and it also has a knocking sound at low idle. I examined the blade clutch and noticed some rubbing between two of the parts (see photos). Everything is torqued right and I also checked for a bent crankshaft and it's straight. Possibly this knocking sound is coming from the top of the 'driven clutch' rubbing against the bottom of the 'ball control plate assembly'. I don't believe the ball control assembly is supposed to be scored like it is. The 2nd picture is of a new engine, and there is a 1/16" gap or so between the 'driven clutch' and the 'ball control assembly'. I will look into possibly putting a shim between the two parts.