Noisy heating system

[barrowfordred]

My fairly old (only moved in 6 months ago so I don't know how old) heating system makes quite a bit of noise. It's more of a trickling water sound than a gurgling and seems to come from the water cylinder.
If I release the air vent, air comes out then water. If I leave it a few minutes and repeat the procedure more air comes out then water.
Repeat ad nauseam
The tank in my attic has water in it. It it just a case of letting out more air each time from the vent and then waiting for more circulating air to collect? Or is there something else? I've tried bleeding the radiators and no air comes out.
Hope this makes sense.

 

[barrowfordred]

Further to this - If I keep bleeding the vent every 10 minutes or so I can eventually expel all the air and the system is quiet for a week or so then I have to expel more air.
Is there an easy method of replacing my manual air vent with an automatic one?