We have just installed a CGH system and changed the pipes to be mains
fed. This has caused a very loud hissing sound to start coming out from
all the taps when used.

My plumber has said that this is happenening as he has joined 15 mm new
pipes to the existing 22 mm pipes. To change the whole pipe work wil be
costly.

Please could someone provide a solution in terms of changing the taps
etc so that the noise is subdued etc.