Trickling/crackling noise coming from wall?

I live in a pre war apartment in NYC (if construction has anything to do with it) and starting from yesterday I have been noticing a very odd noise which, to put it as best I can, sounds a lot like water in the process of freezing. It starts out like a trickling noise and then crackles like whatever was in there just got sent to the ice age. At first the noise happened maybe once an hour or two, and now it's happening much more frequently, every couple of minutes. As best as I can tell, there is no obvious correlation (doesn't occur when I use water, when someone else uses water, when I bang on the wall, etcetera). This is the first winter I have lived here, so I'm hoping it is something related to the heating system. Any ideas? I don't notice any water leakage or discoloration or anything else visible, it's just the sound that is driving me nuts.