TBH I think I might know what they mean - I kept hearing a very low frequency hum for certain periods and I have come to assume that it is probably our neighbours' either washing machine or tumble drier. I assume the dryer because it doesn't align with the spin cycle sound I can extremely rarely hear, and because of the length of time it happens for (about 2 hours at a time).
The hum of the tumble dryer is unbearable because although it is quiet, there is something about the frequency of it and the constant-ness of it that bothers me, the only way I can describe it is you know when people say "It's so cold I can feel it in my bones" - I can "hear" their tumble dryer in my bones, so it feels very loud. (I completely know this sounds insane!) It is similar to things like traffic noise.
I can also hear our own tumble dryer in the same way, although it's much less loud and doesn't bother me - I think it must be some kind of vibration thing, because putting it on quiet mode doesn't make any difference to this part of the sound.
I haven't complained to them because DH and DS1 can't hear it, and I honestly think it's just me who is sensitive to it and it's perfectly reasonable for them to use a tumble dryer anyway. I try to put headphones on and listen to music when it happens, which sort of works, or I get up and walk around the house, which seems to make it less bothersome.
The only thing I could suggest which might reduce it is one of those foam/rubber mats you can put under the feet, it might be that it's transferring somehow through the foundations rather than through the wall? But that said, we have this on ours since we used to live in a flat, and as I said I can still hear a similar sound from our own dryer, just not as loud.