The disc or the cage inside the valve (depends on the mfg of the valve), is loose or has become unconnected from the valve stem. 1st, try to turn the valve all the way open (CCW). Remember, on a 1 pipe steam system the radiator valves can only be fully open or fully closed., no half open. If this does not quiet the valve you will have to remove the top of the valve and remove the disc or cage assembly that opens/closes the valve. I would have someone qualified in plumbing do this. Yes, with this old piping, you can break fittings and the piping, so I would wait till summer and contact the owner of the property if it is not you, and the building manager.
If the plumber knows what he is doing the fix should go OK. Anytime I fixed a valve like this, I used a back-up pipe wrench on the pipe to lessen the torque on the pipe. You hold the pipe wrench any way you can and tap the wrench on the top of the valve with a 3-4# hammer to snap the bonnet loose. Worked every time. One more thing, if you want some temperature control on rooms that overheat, you can add 1 pipe steam zone control valves to that radiator. They work by controlling the venting of the radiator. If you do this, the thermostat controlling the boiler should be moved to the coldest room and that room does not get a radiator control. Do a search at Supplyhouse.com or Danfoss.com. I like supplyhouse the best for their information. Let me know if you desire more info. There may be newer products but I have been retired for 11+ years. hope this helps.