what are the silver/grey boxes for?

they are motorised valves they open and shut. when you turn on your heating stat it sends a signal to your valve, pump, and boiler to come on so your valve will open to let the water through. one is for heating and one for hot water.if they are grinding they may be worn.
but unuseal. you can take the top of them and have a look, its only a motor and some mech inside. get someone to operate the stat and watch whats grinding inside.

hi-spec plumbing
March 2007