RV been sitting this summer, so I was checking both roof AC units. Front one was fine, but rear unit wouldn’t start—only hummed. Last summer I had replaced the caps and installed a soft-start cap, and it was running fine. So up on the roof I went to investigate, thinking maybe a new cap failed. As I was about to lift off the cover, the thought of bees entered my mind. After carefully moving it away, I spotted the problem—a large hornet nest was built into the fan blades, in such a way it prevented the fan/motor shaft from moving, hence the humming.. Since this was just after dusk I didn’t see any activity, luckily. I had carried some wasp spray up with me, but decided to go back for some brake cleaner spray. I then saturated the nest, poking and prodding to get the spray into every crevice. Oddly, I only saw one or two bees fall out…the nest proved to be quite an ordeal to remove but finally it all came out after repeated tearing at it with a long stick, and a lawn blower. After that, the unit ran just fine. Whew! (I was a bit more nervous than usual having taken a nasty sting to my wrist a day before from one of those giant “yellow jacket” types)