So I was having this intermittent tone problem, and it was driving me buggy. All summer long I kept playing with what could best be described as "diapson" or two tones at the same time. Didn't sound terrible, just terribly different. I hated it. As fall approached I noticed the problem was getting more intermittent. One day I was getting hot while practicing and got up and shut the ceiling fan in the room off and my "diapson" magically disappeared! DUH! Apparently the sound while playing was echoing off the fan blades overhead and I was hearing the change in tone with each fan blade as it passed.
I bet I am not the only one who has experienced this....but I spent a LOOOONNG time working on my tone because of it! Silly me.