This is both a plant and insect question. I manage a property that has a small stand of planted English and Garry oaks. This summer, there has been a very large population of bees and yellowjackets attracted to these trees. There isn’t a nest or hive on the 5 acre property that I have found, so these bees seem to be coming in from a distance. Indicators that I am observing are:
Many sap (sucker) holes on these trees, one in particular.
A heavy black dusty coating on most of the leaves.
The sound of droplets, not humming or eating.
Based on your description, my first thought is that the coating on the leaves sounds like sooty mold. First comes the sticky honeydew left by leaf-sucking insects such as aphids, and then the sooty mold develops on the sticky substance. It would make sense that bees and wasps might be attracted by the honeydew, but might also be preying on the smaller insects that cause it.
This article, which comes from a beekeeping perspective, may also be of interest. Here is an excerpt:
“Although it may be off-putting to think of eating insect excrement, honeydew honey is prized in parts of Europe and New Zealand and often fetches higher prices than floral honey. It is rich in mineral content, amino acids, and may have stronger antibacterial properties. When a psyllid insect or aphid ingests the plants sap, it digests the small amount of protein present and expels the rest of the water, sugars, tannins and other indigestible material as honeydew. If you have ever parked your car under a tree and found it covered in a sticky substance the next morning, chances are the tree is infested with sap-sucking psyllids or aphids. For many other insects including ants, wasps, and bees, this is a valuable food source.”
Your poetic description of “the sound of droplets, not humming or eating” might be the sound of transpiration from the leaves, or just the honeydew (part tree sap, part insect excrement) seeping from them.