...Is this normal? It's worse in some radiators than others, and it stops as soon as I turn any hot tap on. I don't remember this happening in the past. I'm concerned because we had a problem in the summer with the radiators spontaneously coming on when the hot water taps were turned on (even though the heating was switched off at the boiler.) The repairman corrected that problem by replacing a couple of things including the divertor valve(?) and this rushing sound is now making me wonder if there might still be a problem. Surely there shouldn't be any water actually going into the radiator when heating's on,(aren't they already full of water?) and why would the hot tap stop it?? Huge thanks for any advice anyone can offer.