I wasn't able to down load your wav file. Is it a constant hissing, or are there periodical blasts of hissing?

a lot of constant, there are blasts of hissing too, but the constant hissing is so loud that it's not materially better than the blasts. The blasting sounds like a pressure cooker, it's more like uneven hissing

I think you have to be logged in to download the wav file.

Thanks for the tip. I'll look into it.

The super said some hissing was because he had to have high pressure to get the heat up to the high floors.

He changed the valve for me and said there was water pooling inside of it which probably made the loud uneven sounds. It was definitely the only radiator in the apt making the loud uneven sounds, so hopefully it'll be a more even hiss from now on. If hissing is indeed inevitable from the pressure, at least it hopefully won't wake me up at 5:30 am every morning like it does now.

How'd the new valve work out? Steam systems are hard to get working well, but when they are they should not sound anything like your sound file!

Changing the valve solved the problem. But I also suspect the super may have changed the pressure after hearing what it sounded like in our apartment. And there may have been some changes going into the big snowstorm that have made the heat steadier than before. All I know is the heat is way better now. No more waking up at 5 or 5:30 am to that loud hiss.