Most likely air traped in the system. If you have the bleed key you can try cracking it a bit and letting the air out....If that does not help the regi needs to get the stuff fixed. 

https://www.youtube.com/watch?v=YR97NZdVWbY

This happened to me when I changed flats. Could hardly sleep.

Continue to contact the regie.

i kept wriiten records of the time, the frequency, what the sound is like to show them when they come. It gave me something to do as I couldnt sleep and was recorded eveidence of the problem.

 I asked someone help to write me a letter to the regie complaing of hammering sounds late at night and early hours of the morinng sometimes almost non stop during the night which prevented me from sleeping.

My regie  fed up of me  I suppose,  and now with it in writing, eventually sent the central heating company who came and l hummed and haa-ad a bit. I shoed them my records I'd wriiten down aabout the noise I think 3 people came separately...  The central heating company were half scared to do anytting (!) because now the responsibility was on them. Were worried what ever they did would affect their future relationship with the regie

in the end, half terrified it wouldnt work and it being a bit extreme, they reluctantly drilled around the pipes, ceiling and floor, so the friction of the wall, or whatever, on the pipes stopped and so did the noise.

sorry aboiut the typos above.. hope it is clear.. keyboard playing up,.. was typing too quickly..

Very interesting. I rent an apartment from Tournier Regie at that postcode (1205) and have that exact same problem.  I have complained numerous times and a company Benoit have visited and played around with the pipes but which has done nothing.  Interested to know if more people are experiencing the same issues

If they don't sort it out go to ASLOCA and ask for advice and  how to proceed. 

Hi Ben,

What I ended up doing eventually was going to ASLOCA. They helped me write a letter based on which I announced the Regie that on a certain date in the future I will stop paying rent to the Regie, and instead pay it to the local justice body in charge of such litigations - and that body will then need to make the Regie prove that they fixed the problem in order to receive their money. 

The minute I sent them this letter - they immediately reacted in a very different manner, the issue got escalated to a more senior guy who sent a proper company. They made a hole in the wall and realised the pipe leading to the room-radiator was touching the main vertical pipe of the building. So what they did was simply to put some rubber in between so to separate them.

Problem solved now :)

Good luck!

Dan’s experience proves that asking ASLOCA for advice is the simplest and quickest way to get  Régies to undertake any necessary maintenance work that is their responsibility.