rushing sound in hw cylinder

[joe22rm]

i have a rushing water sound in my hw cylinder also water running out of expansion pipe in loft, level in tank constant.
every few mins a rush of air seems to flow round the system.
the water increases coming out of the expansion pipe as i turn up the pump but the sound of running water in the hw tank is confusing.
any help would be good

 

[ricarbo]

Perhaps the boiler is overheating and boiling the water. If you bought a pipe thermometer fro a plumbers merchant you could check this, or you could turn down the knob on the boiler to see if it fixes it.