Why is my radiator hissing?
This is basically going to be landlord work unless you plan playing heating technician and checking the basement pipes. Here's the gist:


Air vents on the radiator must flow air out so it can be replaced by steam to heat. They sometimes have adjustment valves on them so you can balance the system.
Air vents that are producing a lot of noise are either

too small of a vent
clogged up / broken (this is you because you're leaking steam)

Improperly designed systems can also push a lot of air out of the radiators. The steam delivery pipes should also have air vents on them before the radiators.


When working properly, those air valves should not let steam through. They should close when the steam reaches them, and open when there isn't steam. If there are showing visible steam, they need replacement.

I found this to be a good article on caring for steam vents that will provide more detail.