water noises around pump area

[steve1000]

I've had a plumber flush through my 20 year old oil fired central heating system [which has always been reliable] & refill with inhibitor. There is a lot of 'rushing' 'running' 'pouring' water noises sometimes now, around the pump/valve area, when the system is working. It sounds like air to me. The plumber [local, reliable?] has come back, scratched his head & walked away trying to tell me it's normal admitting he has no idea what's causing the noises!
I do have a 10 bar vent device fitted beside the cylinder, which appears to be working; I took it apart & cleaned it, there is an occasional slight hiss as if some air is coming out. The radiators are bled regularly & there is no air in them. I'm a bit stumped; any ideas please as to what may be wrong?

[stoneyboy]

steve1000,
If the pump is feeding downwards from a vertical pipedrop there may well be slug of air in the system. Try turning the pump speed up and see if this clears the problem.
end

[htg engineer]

It's air, the system has been drained and refilled - try venting pump. make sure auto airvents are open and bleed radiators every couple of days - it will clear eventually.

htg