Worcester 24i Junior Boiler Fault

So this is happening when I switch on my central heating. The boiler ignites but after making a huge sound it gives up.
Pressure seems fine and I am still getting some hot water but no heating. If the heating is left 'on' the boiler repeats this process about every twenty minutes but always fails to ignite.
Any ideas what the problem could be?

[@chessguru900]

Juts open the vale underneath the boiler with the plastic key turn it anticlockwise and bring the back needle top level to the red. On mine heating works but no hot water.

[@RashmikaClavain]

Requesting permission to unclench sphincter.

[@smac812]

Interesting our boiler sounds the same, though it was a air lock in the system, turned out to be the water pump worn out....👌

[@chrishardy1443]

So can you please tell me what is causing this noise is it the p r v under the pump

[@bobbylawrence81]

And get some pressure in the dam thing👍

[@traian23us]

Central heating clogged, power flush

[@chrishardy1443]

Turned out pump faulty