if it sounds like a "howl", take off the top instrument cover and look to where the wires go through the side to the mirror. Mine had a 1"+ hole that howled in cross winds... filled it with expanding foam. If not that. You can try putting the furry side of Velcro on the vertical edge of the sliding window glass where it goes into the channel, to force a better seal.
Another thought would be to apply tape onto outside, small area's at a time. Then drive to isolate the problem area (I guess windscreen as well). There has been mentioned problems with "Weep holes" intentional designed in the frames of the side windows but I have not found them on my MH...