Hello all, got a new sound that started a week or so ago with an '81 GL500. As I decelerate and clutch into neutral, engine rpms drop enough that I can hear a mechanical "moaning" sound whose pitch drops with the speed of the bike. As I stop, it disappears. This is a new sound, one that I hadn't heard before. It's not possible to describe the sound except in this manner. Does anyone out there have any idea what might be causing this? All fluids are fresh and full. Outside temps have been in the 30's and 40's, but I've ridden in those temps before without this sound.