Thanks for the advice. This noise does not follow engine speed. It goes away completely as speed increases. At this point, you can turn the volume down without the noise returning. Then, when I slow down, the distortion returns. If I turn up the volume it stops until the I slow down even more and then returns.
The distortion is present as you turn the volume knob from first sound to about 10 detents (clicks) of the knob. When you pass about 25 mph the distortion stops, sound is clear and you can decrease volume by a couple of clicks before it comes back in. When you pass 40 - 45 mph, same scenario of clear sound with ability to lower volume by a click or two. Distortion is always present at volume lower than this setting - about 6 clicks.
This is why I was wondering about speed sensitive volume. I just can not find any reference to a control or setting for it.
Again, thank you for the input.

tl