Humming/heat?
I once accidentally dug up a whole nest of underground dwelling wasps with a skiploader. One scoop, and they went crazy.
I jumped off the loader and ran like crazy. Although I was stung about 30 times (and looked like Worf from Star Trek for awhile), they attacked the tractor savagely.
A whole cloud of them went after the engine and muffler. The humming and the heat.
I'll bet you're right, Bjarney