I've never had a radiator before, all central air for me, but I'm living in an apartment that has them now. All of the radiators in my apartment have been hissing, and pouring out steam from the "valve" on the side. I'm not sure what the "valve" is or even if it's a valve, but it seems to relieve pressure and it's not always on when the radiator is so I'm guessing there is an 'on' and 'off' position. Here is a a wikipedia picture of one that has the same type of "valve":
My questions are as follows:
Should it be hissing? (it's really loud)
Is there anything I can do to make it quieter?
If there isn't, would an unorthodox solution, such as putting something that allows air through over the "valve" be safe? I've used a wet sponge for short periods when it really annoys me.