Hissing radiator for 12hrs when switched off

[kerry452]

Hi all,
Ive had my combi boiler serviced this morning with no problems during that time, no hissing from any radiators, put on my heating for about 2hrs then switched it off and now since then i have had a hissing sound exactly the same noise when its switched on on just the bedroom radiator no other, its defo off at the thermo and no heat coming through.
Does anyone know what the problem could be? never had any sound from my radiators before when off.
Thanks
KR

 

[Armo74]

What's the pressure reading on the boiler?
Did he vent any radiators?
Sounds like the vent could be slightly open.