My hi flow also suffers from noise when the hot water side kicks in from cold, ie morning start up, it has a sound best decribed as a 'creaking ship' and this happens for about 2-minutes, it makes a noise then goes quiet in cycles of about 30 seconds, the noise reduces and then goes away.
Sytem pressure is OK, i have replaced the pressure vessel, its had conditioner in the system from new.
If i was a betting man i would guess that either the hot water heat exchanger or the main one was clogging up and the noise i hear is cavitation as the more i increase the system pressure the less it becomes, but i am getting towards two bar and its still noisey, anyone had the same .
Had plumbers look at it but they just chaged thinhs at great expense but never made a difference
any ideas
tony, February 2010