No idea, I'm afraid.

Does the noise bother you? Do you want them to stop making it? If that's the case, then go upstairs while the noise is happening and ask them nicely to please stop making it. Or only do it earlier in the day, or whatever solution will work for you. "I don't know what you are doing, but that noise I mentioned last week? It's happening now. Could you please figure out what is causing it and stop it? Thank you."

Bear in mind that in some apartment buildings, noise travels strangely. What seems to be directly overhead might actually be on one side, or upstairs and over one unit, something like that. But if you go up while the noise is happening, you will be able to hear if it is coming from that unit or not.

Or you could just live with it. Once I had upstairs neighbors who apparently dropped a bowling ball on their living room floor every night about 10 pm. Big thud of something falling, followed by a rolling noise. I never did figure out what it was, but it wasn't annoying enough to bother with trying to get it stopped.

camlan, yes, it is very disturbing. I didn't want to knock on their door since it started at 9 p.m. which I considered late in the day but I guess I don't have a choice.

Well, yes, 9 am is late in the day. But that's for a social call. If a neighbor is making noise loud enough to bother you, there's nothing wrong in confronting them about it at any hour of the day--2 am is fine if they are waking you up.

Just be polite, but firm. "I've asked you about this noise before. I'm hearing it now. Are you doing anything that would cause this loud buzzing noise?" You may be able to hear it immediately when they open the door. Or both of you may realize that the noise is coming from some other apartment.

But if they claim to know nothing of the noise and you are pretty sure it's coming from their unit, the best way to deal with it is to ask them while the noise is occurring.

When you are sure that the noise is theirs, if they still refuse to do anything about after you have asked them nicely to stop or move it or deaden the sound somehow, then you move on to the landlord.

Oh, and document everything. Every time you hear the noise, and every time you speak to the neighbors about it and what their response is.