I get it, this is what happens when you have dwellings on top of each other, the two previous people living below didn't mind, this one gave me a note.
I ran indoors frequently but always did it during the weekday working hours to avoid irritating neighbors. When I got the note I went down and asked what time would be good (it's a couple and the one who didn't complain answered the door), I was told if I run before x time nobody would be home. So I did that. A month later another note. I responded and said I'm perfectly fine with running in a gym but is it possible that there isn't a single time when someone isn't home (they are young)? I have a few thousand invested in my home virtual setup and would hate to lose it entirely.
Never got a response to that note or a following one. So I was thinking of writing again, this time to say that I would run indoors just once a week during the day for about 45 minutes and that I'd appreciate feedback on what time is best for them. I'm sure I won't hear back. I think this is a reasonable compromise, no?
My treadmill is on a thick pad which is on a rug, I can add layers (I tried industrial strength 4" noise isolation rubber pads that they use on washing machines, no luck) so that's another option.
P.S. My neighbor has a dog that barks often for many hours during the day, I never complained and didn't want to use that as leverage.