Worst white noise ever

About a month ago, we had some new downstairs neighbors move in. They've got a stereo and surround-sound speakers for their television. Bass has been known to rumble our floor more nights than not, but usually, by 10 or so, all is quiet. Except for one small thing.
They've turned on their master bathroom fan, and left it on. Day and night this humming, sometimes thrumming, pounds itself into my head. I try so hard to block it out, and sometimes I'm really able to, but the constant low-pitched hum is driving me crazy.
Then, yesterday morning, like some gift from the gods, the fan was switched off. I stood in my bathroom, still and breathing slow, in order to enjoy the great silence of no fan. I rejoiced, and rushed right out to tell Brian: "The fan is off! The fan is off! They've finally turned off their fan!" But late last night, my joy was dashed. The humming, thrumming had returned, and was again ruining the nice quiet of my bathroom.
"Why are these people keeping this fan on?" I wonder. "Why isn't the fan noise bothering them, too? Why is their bathroom air so stale? Why can't it be a louder and more annoying noise so that I can be justified in calling the apartment management?"