Welcome to the forum.
There are several threads discussing a high-pitched, resonance type vibration that is caused by the metal tubes in the front grills of some moho's. You can use the search engine near the top of the page to find those threads.
Best way to eliminate that as a cause is to simply remove the grill and hit the road for a test drive. If the vibration goes away, then you have to dampen the metal rods in the grill to prevent the vibration. If the vibration does not go away, keep eliminating one item at a time until you figure it out. You may search the Winnebago forum for solutions, also. Let everybody know how you solve the problem, that is what this site is all about.