[888heaven]

AGE996
Does it happen all day or at night,the mains water pressure does vary according to consumption.
water pressure vary's region to region and the time of day it is normally high late at night and early afternoon when everyones at work or a sleep at night.
it could be your ballcock shutting off too quickly or your automatic washing machine valve.
the knocking noise is water under pressure hitting a dead end ie a valve stopcock etc.
you could have a blocked pipe but I think you would have noticed that.
hope this helps
ian