I wondered if anybody could give advice on the following please? we have a promax 24sl boiler fitted less than 12 months ago.the boiler was fine over the last winter and this summer ,but since turning on the central heating it has developed a really load whistle that turnes into a howl when the boiler fires up fist thing in the morning,it does not do this at any other time only the morning.we had a potterton engineer round but he came at 2pm,and there was no noise he replaced the gas valve as a matter of course,but this has made no difference,he also said that he would not replace the heat exchanger at all because the plumber that fitted it did not fit drain valves above the boiler and it would take too long to drain the system,is this correct? any advice please before the warranty runs out and it is so load it waking the neigbours at 6am.thanks