We bought add-on bug screens for the furnace exhaust when we picked up our new Big Country. Only had it on the road for 2 weeks the first year...and when we went to use the AC, there was a big racket. Took it in for service, and got it back today with the explanation that mud dobbers had built a nest on the AC fan blade while the trailer was in storage; the initial clatter of the AC was throwing off the large nest, but the residual on the blade caused it to wobble...more noise. Service advisor told me that he's seen mud dobbers in water heater exhaust, refrigerator exhaust, etc., and that at $109/hr labor it's usually cheaper just to replace major appliances when mud dobbers come to roost. Does anyone make finer screens for AC and other heating vents...or does some one have some simple fixes? Only thing that comes to mind is some metal window screen....