Loud noise from hot and cold taps

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.

You may have extrememly high water mains pressure, in which case you might benefit from a pressure reducing valve, as long as it's put in the right place!
No need or point in changing any pipes, just get pressure/flow under control.