for his heat pump to making that kind of racket unless there is something wrong with it. Having also experienced the irrational neighbor (who thankfully moved on about 10 years ago) I can say that there will be no pleasing this guy, so don't try. I would do one of two things:
1. Pray the darn thing breaks down completely, and soon, and then he'll have to have it fixed.
2. Call the city and find out what your noise ordinances are. Most cities have them, and they are not just for barking dogs and car stereos.
He will probably think the neighbor he already doesn't like did the calling, so just let him think that, but get the city out to test.
As to your last observation about it being unfair that people are forced to deal with this stuff, I would have to say that they are NOT forced, they choose. Sometimes you have to stand up to a bully, and that's what this guy is. You have as much right to enjoy your property as he has to enjoy his. He probably still likes you because he realizes you haven't figured that out yet. Get tough.