I'm calling the landlord on Monday but thought I might get some information here as well.

I've noticed that some of the radiators are making loud noises. It sounds like running water. If I turn them down a little, it stops (bit cold to have them that low right now though). Sometimes, one will make the noise for a day or two and then stop. But a couple of others have been going for the last week. Still others make no noise at all.

Is this air in the pipes or something?

If it is your maintenance man can easily bleed the radiators to remove any air in the lines.

There you go. vwild said it all. There is air in the line. Some of them you can do yourself if you have the right "key" (more like a little wrench). But be careful as when you bleed the line steam will come out. Don't have your face or other body parts directly there.

Here's a post I did a while back illustrating how to bleed underfloor heating systems (similar for radiators).

I hate steam heat

If you bleed the heating yourself i recommend you hold a tissue or cloth over the bleed hole as when the air is all out (its not steam) black filthy water will come out with force, over the wall or you!

Update for anyone interested.

Maintenance guy bled them all today (yes, it is air). He then explained how we can do it. He said we need a wee key that we can buy at Jumbo and can do the radiators as needed (usually around once a year but can be more).

Also thanks for the tip on black water as my husband proactively moved the curtains out of the way. And yet another thanks for the link to the earlier post, which mentions that this problem is most common in upper floor flats (we're top floor, so good call!).

Ahhh..as much as I enjoy the sound of a waterfall, the silence is bliss. ;p