Wasps
Lots of wasps around at this time of year, you can tell them by the way they buzz, not as frantic as a bluebottle, but more so than a bee. Have you noticed how the frequency of the buzz goes up and down as they fly around the room, I wonder if it's due to the doppler effect or the frequency of the wings. Let's assume the wing frequency is constant and use the doppler shift to find the speed of a wasp. I found a recording of a wasp on youtube and used audacity to measure the frequency of the lowest and highest sound.
highest 141 Hz
Lowest 133 Hz
So the wing speed at rest is likely to be the average 137 Hz
The highest note is when the wasp approaches so f' = fc/(c-v)
Rearranging gives v = 9.7 ms-1 about 35 km/h. According to google the Asian hornet can fly at 40 km/h and a wasp is something like a hornet so it does seem possible that the change in frequency is indeed due to the doppler effect.