If the humming is coming from the front of the coach, I'm baffled too. I was going to say it may be the tires, or wheel bearings, etc. but that wouldn't make sense if it completely goes away when throttle is applied and you have a pusher.
Does it happen at all speeds? Does the hum intensify at certain speeds? ...or change pitch?
Is it an electronic-sounding hum? Does it sound as if it's coming from underneath the dash? Or near the throttle treadle itself? Or does it sound as if it is coming from outside underneath the coach?
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