Our 2002 310 (#175) makes a noticeable "chattering" sound when underway via engine. It has something to do with the drive train, since the sound is absent when in neutral. It also is less noticeable at some engine speeds than others. I've checked the obvious stuff: transmission good on fluid, prop is not loose on shaft, doesn't appear to have any play in shaft at cutlass bearing/strut. Engine only has 300 or so hours. Any ideas?