The converter you have is old technology and the buzz is typical. Replace it with a modern unit (search the forums) that is more efficient, lighter AND quiet.
"Not all who are laundering are washed" say Bill & Heidi
'78 Excella 500,"The Silver Pullit". vacuum over hydraulic disc brakes, center bath, rear twin. '67 Travelall 1200 B 4X4 WBCCI 3737