2003 150 Optimax Started Knocking

[benjineer]

I've only had it a few months,but it is very clean with seemingly low hours. Seller said 190 hrs. It had an overheat alarm the first trip to the lake. I replaced thermostats and impeller and that was gone. Almost perfect operationup to this point. I've run about 60 gal of fuel through it. I had a water in fuel alarm my last trip and dumped thefilter, alarm went away. The enginenever had so much as a hiccup. After filterdump, I pumped it up and ran on the hose for 20 min or so to refill thesystem. No issues at all. A few weeks later I put it in the waterand crank it up and get a little white smoke and a knock. Smoke goes awaybut knock doesn't. I pulled all plugs except the bottom one that's hardto get to. They were blackand dirty but no bits of metal or noticeable problems from what I couldsee. I ordered a new set to rule thatout. I’ve read the threads about the aircompressor. I don’t think it’s comingfrom there. I can hear it the loudest onthe intake housing, but that could be just because it’s a big amplifier. I will take it to someoneand pay $$ if I have to, but I want to do what I can myself. Has anyone here experienced similar problemsand can offer advice? I read an oldthread from 2014 on a 225, but he didn’t post the resolution.