Radiator Loud (Whoosing/Hissing Sound) when on 4/5 on thermostat, but quiet 3 and below?

My instinct about this is that it is going to be a faulty valve.someyhi g about the way that your TRV is opening and closing will be causing this. If you are a competent DIYer and are confident in taking on small tasks then I would suggest removing the TRV head and gently seeing if the pin is moving freely. If what I'm saying doesn't make sense then I would urge you not to tamper with the TRV at all! I think that the closing mechanism of the TRV has a loose component and this is vibrating in the water and it's being amplified by the pipework. There isn't much that can go wrong with a radiator. If you actually had air rushing out of it then it would be closely followed by water once the air had been forced from the system. Clearly there is no way for anything to escape because there is no leak and the system is working fine. So you are hearing the sounds of something happening inside of this closed system. My guess is that it's from the Thermostatic radiator valve, this assumption is further supported by the fact that operation of said TRV has a direct impact on the noise. I would get someone to replace it. Thanks Deke