1989 Yamaha Pro V 150 running a 23-M2 Yamaha Pro prop. Everything is stock. I can get to full speed without a hitch but when I slow down to cruise the sloughs at ~3100RPM's to ~3500 RPM's, I hear what seems like a baseball card running through spokes from the engine area or something slapping in the wind. Noise is not a metalic sound at all, but like a thumping or rubberized sound is the best I can describe.

But, when I punch it to over 4000RPM's to 5600RPM's, the noise goes away. Also goes away below the 2700 or 2600 RPM's too. Or I think it does as there's a lot of wind turbulence thereby not being pronounced..... I verified that the fuel pumps are tightened (thought maybe sound was coming through vacuum holes for the diaphrams). Also checked plugs. They are normal. Motor starts and runs fine, hasn't lost any power or punch, and idles really well. I'm thinking check valves in the primer bulb? Or the prop spinning too fast for the speed, or a Lower Unit problem? I have a 25M prop that I'm going to try and checked LU oil for metal parts. Nothing found! Also, could the reeds make this type of noise? Or carb butterflies?

Any ideas or experiences?

Thanks in advance!!