sounds like water hammer, caused by sudden pressure changes within the pipework resulting in sound waves - maybe worth looking into some type of air trap to add to the pipework to add as a damper
check this article
http://en.wikipedia.org/wiki/Water_hammer
yes, it has an immersion heater but there is an issue with the water supply from a previous tank which serves the whole building but the building is part run down, part vacant and there is ongoing problems with the supply from this tank.
Thinking a tank within the accessible part of the ceiling...
How do you pressurise a water expansion vessel in au unvented hot water system and how do you know you have set the correct pressure.
Always been interested to know this.
Thanks
p.s. no need for 'ask a plumber' replies ;)