air conditioner "buzz"
While vacationing, we tried to run our air conditioning and it sounded like there was something there that the fan was brushing against. We figured it might be a twig, so we turned it off, and when we got back home, climbed onto the roof and removed the housing, to find a HUGE hornet's nest. Luckily, the nest was already vacated but a few bodies and larvae remained, which we vacuumed up.
Not recommended. I had to take the vacuum apart and clean it.