I have a potterton suprima 30-80 style boiler. My problem is this: Once the CH has clicked off a howling starts and the sound can be heard though the pipework between the boiler and the boilermate. It last for about 45 secs and is sounds like a valve in the boiler is closing or switching flow from the CH to the HW system, that is causing the noise. Any ideas of how to make this quieter? is the valve in need of replacing? 
