Radiator squeals and vibrates after stem valve is shut

I was referred to this site by Reddit and this is my first post so hopefully I'm doing this right.

Anyway, I'm visiting some friends in Manhattan, and I'm staying in the living room of their 5th floor (it's also the top floor, probably built pre-war) walk-up apartment. It's atrociously hot in here, so we got permission from the superintendent to turn off the radiator by closing the stem valve. While the room has indeed become cooler, the radiator has occasionally stated making a horrifyingly loud sound from time to time akin to a loud, buzzing squeal of varying pitches that lasts for several minutes.

I've observed a few things:
1. The sound is accompanied by a relatively high frequency but fairly low amplitude vibration in the the radiator itself.

2. This vibration seems to be the worst around the trap in the lower-right corner of the radiator.

3. I was listening for the steam cycle to begin, felt around a noted: the return pipe & trap were cold, the radiator was cold, and the steam pipe was hot. About 90 seconds later the sound started and I observed that while the radiator was still cold and the steam pipe was still hot, the return line and the trap were both just as hot as the steam pipe.

4. The noise only started after we turned the radiator off and it had cooled down.

Radiator:


Trap & Return:


Steam pipe & Stem valve:


Everything I was able to dig up here or online has to do with knocking in the radiator or related pipes, so my search was fruitless. Does anyone know what's causing this or what can be done about it? My initial inkling was that it's some sort of thermal difference between the bulk of the radiator and the trap somehow related to the boiler's steam cycle and there is naught to be done. Is there such a thing as steam backflow? And considering how dangerous steam can be should I just re-open the stem-valve?

For good measure here is the response I got on Reddit, courtesy of user NorthernUrban
"Steam systems are built and balanced so that all radiators are running, when you take one out of the equation you un-balance it and you can get interesting results. On top of that, the valve you closed might not completely seal allowing some steam to get in and if it's traveling through a small hole into the radiator it might make the noise you describe. You mentioned this is the 5th floor rad, is this the top floor? Typically the top radiators in my house heat first since hot air rises, so turning this one off will give a more dramatic effect than closing one on a lower level. One thing you might want to do is check the pitch of the radiator, is it draining towards the lower pipe? Set a level on top of it and see. Water trapped inside the radiator mixing with steam will give you water hammer.
You can check out heatinghelp.com, they are pretty knowledgeable about steam but I'm not sure there's much to do here except open that radiator back up."

Thanks guys!