wrote

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.

Could simply be that the plumber has opened the water stop cock further
after doing the job than it was when he shut off to make the changes IYSWIM.
Does water come out of the mains fed cold taps faster than before he
started?
Closing down the stop cock a bit may cure it, but may also affect combi
performance - don't know about combis, don't have one!

Phil