It's unusual for a gravity hot water system (low pressure) to cause pipe rattle or water hammer. I agree with the others that the problem lies with the mains pressure (cold).
When you turn on a hot outlet, the roof tank immediately opens to refill the tank. Turning on a cold tap reduces the pressure and stops the problem.
Do what Tony suggests, and turn your rising main (incoming supply under the kitchen sink.) See if that makes any difference. It's not a final solution because it also reduces your flow rate (amount of water coming into the building), and it won't allow for normal variations in the mains pressure.
If that does it, then try fitting one of these just after the incoming main.
http://www.ebay.co.uk...0150678463#vi-content