Why don't you go introduce yourself, and ask them?

Air compressor is a good guess.. What would a family use it for though? They keep whatever this device is.. in their bedroom closet. It's gone on and off about 4-5 times since I first started this post tonight.

I don't know but I understand how irritating that would be for you. I would be a nervous wreck in the same situation...

I woke up at 12:30 am, 2 am and 3 am hearing it last night. I haven't heard it yet this morning after waking up but knowing them... I'm sure it will happen soon enough. I'm going to ask management about it.

I think you are right to do that. An apartment is a very unusual place to be running an air compressor if that is what it is.

maybe its forced hot air heat, my friend has one in her apartment and at times it sounds like a jet when it starts running

That's a good guess but it's been in the 60s here, 70 yesterday.

I'm going to guess medical equipment but not acceptable if it's disturbing you. In order to keep you out of it, ask Mgt to send handyman to ck on a noise they heard vs saying they are responding to s complaint. They will understand and go along with it or maybe they already know situation and can explain it tonyou

I just received an email back from management, they are going to give them a noise complaint letter tomorrow morning. They said if I hear it after hours again after that to let them know.

I'm assuming that there was no noise before they moved in, otherwise I'd say it was a heat pump or other hvac equipment turning on.

As people mentioned above, could be medical equipment, such as a loud CPAP machine for sleep apnea etc.

If you are in a multiple unit building, it may not even be them causing it. I've had odd sounds in my condo building before, that turned out to be boiler issues that caused pressure in the pipes and knocking etc, but sounded like it was the unit next door.

It def started after they moved in and if it was a sleep machine, I don't know why I would be running all day as well. It's a very strange situation, it's hard to explain but it will be interesting to see what happens now that they received a letter today about it.

It may be an adjustable air bed (sleep comfort). When the pressure goes down, the pump starts up. Normally they don't run 20 minutes, but if they have a leaky mattress or someone heavy is getting in and out of bed, they will cycle a lot. If so, they can put it on foam to help insulate the noise transmission.

Have you considered asking your neighbors what it could be? It might be a medical machine needed for one of their children.

CPAP machine?

Dehumidifier ?? As I read this I thought OMG, i wonder what mine sounds like downstairs.. I bought the largest one I could find which is also probably the loudest one made. I have a dehumidifier on one side of my bedroom that turns it self on/off and also a very large air cleaner on the other. They really help block out the noise from the upstairs cave people and as a side benefit have really helped my allergies/asthma.

When I first moved in here, there was a loud noise that woke me up several times at night. Turns out, it was the defrost cycle of my fridge, which is very loud. I guess I'm used to it now, because I hardly ever hear it anymore. I used to live in an apartment by a train track where a train went through at 3 a.m. every morning and blew the whistle. The train yard and switching station was about a half-mile down the road. I never even heard it after awhile. It used to scare other people that came to visit. They'd say "What's that noise?" and I'd say "What noise? Oh, that's the trains."