They do have to run 24/7 as said. it probably isn't as noticeable to anyone else. And it's not like they are thinking," hear that hum, that guy is growing pot!" Make up something that you are doing that requires it to be on 24/7. Then go ask whoever you are worried about if they can hear your xxxxx. Good chance they'll say no, think you are a considerate neighbor, and you can forget it.