Ok, this will sound goofy so here it goes. I have a 25hp Yamaha tiller prop job in the rear of my 16’ Bass Tracker Grizzly tin bot. I had something vibrating at low rpm’s. Similar to your noise. It went away as I gave it gas. I tightens every bolt, screw, motor mounts, and any nock and cranked I could find.
Next trip out it was gone.
just say’in.