Thorn M54 Boiler Noise

[conundrums]

I have a noise problem with a Thorn M54 Floor Mounted Conv Flued Boiler which has an unusual noise senario.
It is in excess of 20years old but in exceptional condition externally. The noise is only experienced upon start up for about 15 mins and the same when cooling down, but is enough to awaken the house hearing the noise through the radiators and is annoying at other times.
The noise has only started 2/3 weeks ago and goes away completely after the initial period unlike kettling boilerswhich tend to build up as they heat up.
This noise is of a surging nature, a rattle in the boiler like the slow rev of an engine.
The only modifications made to the system were new radiators throughout 3 years ago.
Has anybody any experience of this type of surging noise on start up and shut down, any advice would be welcome.

 

[bab]

if u had said it only did it when heating up i would of 1st thought of the pump bearing before it got warm but if its as noisey when cooling down then not sure . how olds the pump & what speeds it on & is the system fully pumped of gravity hot water.

 

[conundrums]

Pump hasnt been changed as we know of and is on speed 3 where it has always been. System fully pumped. Stopped the pump when noise happening and it makes no difference. obviously pump only stopped for short period. Noise is cyclic vroooooooooooom vroooooooooom slowly increasing from nothing then back to nothing repeated for up to 15mins with a rattle present. Definitely seems to eminate within the boiler as if a baffle is rattling until it expands with the heat, and when hot it gets stuck and cant rattle. There is no noticable vibration in the system, can only suspect scale within the boiler doing the same thing but the noise sounds too metallic to be scale.

 

[conundrums]

Still no change, plan to change pump to see if that resolves anything & if not add Fernox Superconcentrate Boiler Noise stuff which will need time to take effect.

Has anyone any other ideas, I will update with progress report.