Sorry to hear about your motor problem!
I think I'm having the same problem. My very faint knock/noise is @ idle only after I have run it for awhile and idle. Is your knock intermittent? My kinda comes and goes. My 1999 HP500 Carb. and has about 73 HRS on it. My motor runs great, although, this weekend when I started it, I got a little puff of blue smoke when I first started it. Let us know what your mechanic finds out. I'm getting more concerned about this now after hearing your problem.