Draining radiator - what the hell is this noise?!

[hashamyim]

So, decided to change out our radiators, therefore I need to get the water out of them. Fine.
I shut off the water-out valve at the bottom, then shut off the water-in valve at the top. Wonderful.
I then loosened the nut at the bottom of the radiator in order to slowly let the water leak out into a bucket. Fabulous.

But then, every time more than approx. 500ml of water comes out, this noise starts eminating from the water-in valve / temperature regulator handle. It sounds much like an unhappy 3yo child.
I assume that it is caused by a pressure difference, because it stops when I close the nut and open the in-valve.

Sound recording:

Does anyone know what this implies? Does the valve need changing?! :-(

Regards, Hashamyim

 

[Geofd]

Is there a vent on the radiator????? Loosen it to help draining them use a wet vac against the union so it's faster and less messy to drain no sound from the video