We've been pleased with our Worcester 36cdi installed in 2013. Over the past few years we noticed a low droning or rumbling noise which gradually increased to the extent that it woke us up in the mornings and pervaded the house whenever the heating was on. Worcester were very helpful and sent its technicians on two occasions, changing pump and fan, but it made no difference. I should have noticed that the rumble only happened when the burner was operating, the fan and pump produce only a faint hum. As our boiler is fitted in the former hotpress on the other side of a drylined wall to the hallway, we stripped the wallpaper, removed the outer skin of plasterboard, packed it with a £45 roll of acoustic wool, put it all together again and repapered. It made no difference at all. Last month our gas fitter who installed it failed (yet again) to show up for its annual inspection so we called another fitter who inspected the flue and pointed out that it was not secured as per current regulations, which require each joint to be secured. The flue comprises one metre vertical from the boiler to a 90deg bend, 1.2m horizontal to another bend, and 2m vertical through the roof, this being the only section with a fixing clip to the roof truss. I should add that this was approved by the building inspector when new. He fitted clamps to the horizontal section and to the lower part of the final section. When the boiler fired up that evening, NO NOISE! We're delighted, thanks Gary! My conclusion is that the plastic flue and its seals were flexible enough to absorb vibration but have gradually hardened over the years, so becoming a rigid pipe which vibrated when air was passing through it. From forums such as this I found that this rumbling noise is not an uncommon complaint, so hopefully our experience may help someone else.