Nothing to do with pressure, but for the noise description, well spotted, it is actually known as 'kettling'.
Either the pump is not running correctly, or the heat exchanger is scaled up, rather like a kettle gets, but more expensive to correct.
Try adding an internal de-scaler such as sentinel x200 (there are also other makes) which may help, it's a £15-20 gamble, if it works, great, if it doesn't, you're into a new heat exchanger.
Carefree
November 2010
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