When sailing, there is a low growling sound caused by the shaft being turned by the water turning the propeller. I have a drip less connection, so does the shaft turning create unneccessary bearing ware? I experimented by putting the transmission in forward, but the growling continued. In reverse, the growling stopped, but this would add to drag. What brought this to my attention was noticing the change in sound as boat speed changed. I had noticed this in my last boat also. Is this an issue of concern or the product of an idle mind?