My six year old Powermax has developed a whistling noise, which stops for a while when switched off and back on again. It's normally when cold - once warmed up it doesn't seem to be a problem. When it does whistle, it comes through the roof flue, and is quite loud outside as a result. It also seems to be locking out on a more frequest basis.
Any suggestions? Is it something as simple as air in the system?
Any suggestions? Is it something as simple as air in the system?