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.

Mine does the same, toddlerchaser. It happens right when the thing first kicks on and last for a couple of minutes before it settles down. SInce my F&P is headed out the door, like Monday, I can only tell you could have it looked at, but you'll probably get the "that's totally normal" statement thrown at you. Maybe it is normal. For Fisher & Paykel. But I'm not sure it's normal for all refrigerators. I'm sorry I can't offer more encouragment than that. I'm pretty put-out with F&P right now (which is why I am so EXCITED to be rid of this refrigerator come Monday).

some possibilities. Stuck sump pump. Stuck sewage ejector pump. Stuck attic fan. Lost cell phone or pager. A childs toy. Water meter? Even heard of one that turned out to be the gas meter.

Quite normal. Remember that one of the trip elements in a breaker is electromagnetic. It moves a piece of metal inside to trip the breaker... You've got 60Hz AC passing through an electromagnet with a semi-movable piece of metal in its field... If you've got enough current going through there, there's going to be some vibration! Oddly enough, however, my project GE panel (very nice, I must add. LOVE the neutral/ground bar layout) makes no such noises.