I couldn't see a topic like this one, so thought a new post was best.
It's hard to describe in text, but when our heating is on, just after the boiler switches off and the pump over-runs there is a sloshing/glugging sound from what sounds like the pipes just above the boiler.
Almost like the sound of puring water into a fish-tank type noise.
I've built up a theory that it happens most when it is only heating the water.
Also, one of the radiators (the one closest to the pump in the airing cupboard) needs bleeding every couple fo weeks as it fills with air.
My guess is that when the water only is heated, the pump finds it easier to suck air through the header tank rather than around our fairly large system.
Based on that, I'm thinking of raising the header tank to increase the head of water above, but wanted to get some advice before I do anything.
I should also mention, if I put the pump on speed 2 the problem goes away, but that speed isn't sufficient to heat the rads, again due to the size of the system.