We have a Kenmore 9000 Super High Efficiency. While it's running however, it will go from a quiet, reasonable hum to a buzzsaw sound that will stay on or come in/out, then stop, etc. with no rhyme or reason to it. Almost sounds like a branch is stuck getting ground by the blades but I doubt that's it. It's been doing this for years. The unit is level and otherwise works great.
Any advice on what could the sound be?