We have a 3.5 year old Halstead Eden X Type VBX 30 that works efficiently but it has a loud knocking & vibration period 5 minutes after heating light up. The problem manifests itself for about 5 minutes then all goes relatively silent. It does not happen when hot water only is on. The noise, reminiscent of a low flying light aircraft, is accompanied by vibration in the vicinity of the hot tank, pump & floor in the airing cupboard upstairs. The boiler, immediately below in the garage, also vibrates but less so. We have had it checked by the London Halstead experts who say the heat exchanger has probable contamination & the system requires power flushing but this was done when the boiler was fitted 3.5 years ago. All the rads have been removed & flushed during a major house update session since then. A reluctance to continually power flush is the system age as it is about 28+ years old. I would be prepared to remove the heat exchanger & check it for contamination but although a competent aircraft engineer & D-I-Y'er I am not 'Gas Safe' registered so reluctant to touch it, although I would consider doing so if a registered gas engineer would check it all out & recommission the boiler - but I understand a reluctance for anyone to do so. Has anyone out there any constructive answers to our problem please? I wish to add that in early March 2010 I added two canisters of Fernox F2 Boiler Noise Silencer but to no avail. The system still carried on as before. We have not run the heating at all since the end of March, just the hot water, for no other reason than we did not require it on. I am now concerned that all the system contaminates are laying there & may cause bigger problems when we do switch the heating on again - what is my next move please?