High pressure and banging pipes

[Mark24369]

Ive recently had my annual boiler service.The engineer added air into the expansion tank.In 10 yrs of servicing ive never seen this happen.Straight after the service when the hot water was used a humming noise would echo throughout the house.Call back the engineer who apparently released some air out of the expansion tank but the problem carried on.So he returned and blamed a faulty pressure relief valve.I had to pay £130 for this repair.Since changing the hot pipe doesn't hum but the cold pipe is banging in the wall which is very irrating at night.Another engineer came and checked the pressure in the expansion tank and was astonished how high it was!This engineer has pinpoint the banging to a unclipped pipe.Could this problem been caused by the high pressure in the expansion tank.Many thanks