knocking hot water pipe

[blastedfury]

When we moved into our new house about a couple of years ago the hot water pipe knocked when you turned the tap about a quarter turn the pipe would start knocking then the more the tap was turned the less water would come out. I replaced the tap and everything has been working fine since then. But now the problem has returned. I've taken the taps apart and everything all looks ok.

Could anyone give me any suggestions?

 

[REDSAW]

what system is it,
if its a open vent, hw cylinder sounds like blockage in the cold feed to cylinder.
when the tasp are firs opened the draw of is normal as the cyliner is under slight pressure. after a few seconds water slows down because the cw pipe cant replenish whats drawn of quick enough,
try tapping bottom pipe and cylinder at bottom.

 

[blastedfury]

thanks for the response, the hot water is fed from a combi boiler which was put in about two months ago.

 

[REDSAW]

what boiler is it and the rating.
does it do it on all the taps?.