We are having a problem with some sort of airlock - a buzzing noise. I
live in a block of flats. I believe the problem lies with the rising
main, though neighbours disagree.

The rising main terminates at a storage tank in the loft. I believe
think various neighbours have taken feeds from it over the years. At
the entry to the storage tank there is an vertical extension to the
pipe, about 12 inches long. I *think* that was put there as a
silencer previously. Should this extension contain water or air? Is
there any action to take, such as bleeding it, if we are now getting
water hammer?

I have tried pushing down the ball cock in the hope of releasing air.
This seems to have helped but not fully resolved the issue. Any
advice welcome.