Radiator OMG

[jadey]

The radiator pipes in my sixth-floor apartment have, sometime in the last half hour, started making a very odd noise. Not clanging, but a strangled wooshing noise like water is trapped in them or having a hard time making it through. I live in a very cold city (it's been hovering around -20 Celcius for weeks now, even during sunny afternoons) and I have already gotten the building-wide notices from the building manager about keeping our thermostats over at least 18 degrees to prevent freezing and bursting in the pipes. I've done this, but I also noticed that the pipes are currently stone cold, and at least as recently as 3pm this afternoon they were warmish (had occasion to check while on the phone with my mother). It's Christmas Eve so I really don't want to call anyone if it's just a normal radiator thing and I suspect I won't get hold of anyone anyway, but I'm really nervous about this weird noise.

I tried turning my thermostat up, and the noise got worse. I turned it back down again to just 20, and now there is still a distinct noise of water, but it's not strangled at least. Does anyone with more familiarity with this type of heating system in this type of climate know if this kind of behaviour presages doom? Possibly of the pipes-bursting variety?

Coincidentally, I've actually had people in my flat three times in the last month to inspect the valves (again, building-wide inspection), including by the building manager herself just yesterday! At least, that's what the notices said - I was never actually home. I'm hoping that if there was an obvious problem, someone would have noticed...

Help!

UPDATE: Sucked up my pride and called the supervisor who was miraculously available. It turns out that there had been a leak elsewhere in the building and everything was turned off, and the noise was actually them finally refilling the pipes! So complete false alarm, and thank you to everyone who offered advice.