40 hp Merc runs fine but is super loud only at about 2/3 throttle

I bought a used Tracker bassboat (1999)with a 40 hp Merc motor. It runs great across the entire powerband, but at about 4200 rmps (2/3 throttle)gets super loud. If I increase or decrease the throttle, it quietens down - back to normal. Anyone have a suggestion as to what is causing this? Is it anything to be concerned about? Where would you start looking? Thanks.

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