Gurgling noise from hot water system

[JonBoy]

Hello
I notice a periodic gurgling/"swooshing" noise from my hot water system during operation, even on days when the central heating is not being used. Silent when it is switched off. Particularly audible when standing next to the boiler or the hot water cylinder. Apart from that the system is fine -lots of hot water, no air coming out of taps, no water-hammer.
I suspect it may be air in the hot water system. I know how to bleed radiators, but how does one do it for a hot water system?
My house was built in mid 1970s and has a cold water header tank in the loft above the hot water cylinder. I don't think there is a pump for the hot water system (or at least I cannot hear one in operation - there is one for the CH system however).
I know very little about plumbing so any advice on where to find bleed valves and how to operate them etc would be appreciated. Or maybe it is a totally different problem to what I imagine...

TIA,
Jon

 

[kingkevbo]

try turning on all the hot taps at once and see if any air comes out

 

[masona]

Could be water heat expansion,try and turn down the boiler stat & pump to higher setting (no:3?) then see what happen.

 

[oilman]

It's probably scaling in the boiler which will give rise to gas formation in the pipes. It must be gas of some sort to get the noise.