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Yesterday, after a 20 minute run at 4200 rpm back to the ramp, I noticed my Merc 90 making an unusual noise. Sounded like a belt or pulley squeal, not extremely loud or continuous, but noticeable. I never run the motor hard, and it did the same thing coming back to the ramp about 2months ago. A dozen or so trips in between, it did not happen. I shut it down at the dock, backed my truck and trailer down, less than and when I cranked it back up it sounded normal. My first thought was that the impeller was about shot, but when I pulled the lower unit today it looks like a new one. Stumped as to what that sound could be. The motor has always run perfectly---except for this sound twice now. Any thoughts...?