Why do my TRV's rattle?

The bi-directional TRV's on my radiators rattle loudly when they reach the set temperature for operating (closing). This can occur in any of the radiators/trv's connected to the system which is sealed, pressurised, powered by a HRM Wallstar boiler with Optima TRV's to rads, (one rad does not have a trv and is therefore the bypass route for the heating system).
The rattle occurs throughout the home, any rad, any room, any temp setting and the only way to stop the rattle is to chase around the house adjusting the offending trv slightly until it stops making a noise, this can go on until the boiler turns off or the room stat temp setting is acheived.
I have been advised that bi-directional trv's are not truly 'bi' is this the case, the system is in a new build property and less than 14 months old.