With smaller aircraft hand signals are probably a better idea. Environmental noise is impossible to cancel out, I am sure there are better models than the the standard D&C. There are two types out there, one with a guard that can be strapped to the face (better and more expensive) and the standard boom mike. I have also found that from aircraft to aircraft the feedback changes as a result of the flight interphone amp. Airbus is horrible, when I was on the line I went to measures of taping the foam out of my old hearing protection to the mic because it was very disturbing to me as well.
The only solution is to ask your operator to provide better equiptment to your ground handeling crew, Maintenance Engeneers or AMT's.
Oh on the boom type D&C there is a little white plastic screw to adjust gain but they are difficult to adjust effectively as the aircraft are also a factor.