Have you tried bleeding the pump to see if you can get any air out and cure the 'whoosh' noise? There's a big screw in the centre of the pump which is the bleed screw for the pump.
Sounds like you have a leak somewhere in the system that is drawing in air. With no pumping over, inhibitor in the system and all radiators bled, you shouldn't be getting enough corrosion going on to fill the tops of your radiators.
Thanks, I captured some of the gas in an old beer bottle when bleeding this afternoon, and there was a definate "whoosh" (not a pop) when I stuck a long lighter into the neck. I'm going to do it again tomorrow just to be sure.
Ps; yes, I've bled the pump too. If the gas ignites or pops tomorrow, I'll drain the whole system, flush and stick in inhibitor and see how that goes.
Also, when the heating is on the radiators seem ok and dont need bled, theres always air or "gas' in the radiators when left overnight?
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