Horrendous noise from Bryoni SE?

My Briony SE Combi boiler makes a horrendous shuddering noise when hot water is run from some taps in the house, specifically, mainly the bath tap and to a lesser extent the washbasin tap in the same bathroom.

I have had a guy come round who said it was the heat exchanger. When someone from the same firm came with the part he said it wasn't the heat exchanger but didn't have a clue what it may be.

It gets prtty bad at time and souns as though its going to explode.

Any ideals?