I have a similar problem with two sources. One seems to be the neighboring house, the other is definitely the premises of part of the university hospital, where various fans are located. Both caught my attention for the first time this year. That can really get bad because you kind of "fixate" on such a noise.
Even if your neighbor is a strange character - I would take the tips here to heart, offer him to turn off the pump and test it. Then - if it is the noise - start working toward a practical solution initially with simple measures.
don’t inform!!! Just tell him you did something to the pump and ask if he hears it (turn off at night). If he still hears humming, the humming is in his head.
I had the same! We also have a distance of about 4 meters to the house, there was a cold winter 2 years ago and the unit was running at full throttle. Neighbor had a new girl who constantly heard humming.
The girl is now his ex, the humming is gone.
There is always an external factor. He possibly had visitors who pointed it out to him or something else.