CH boiler - making droning noise - Potterton Suprima 60

I have Potterton Suprima 60 boiler that when I turn the central heating on makes an annoying droning noise that reverberates throughout the house. It does not make this noise when the hot water is on only when I want the rads to come on. The noise will last a few minutes then die away but when you think all is OK it comes back again a bit later. The noise does seem to come from the boiler rather than the pump. I am pretty sure the pump is working as I can feel a slight vibration that increases when you up the setting for the pump. Also the rads do heat up quite quickly.
It started to make this noise last winter so during the spring I flushed the system.
The boiler is not that old. The previous owner of the house had it installed by British Gas 2 1/2 years ago.
Anyone got any suggestions?
Eric.
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Check the fan mounted on the top of boiler,although I would have thought it it would make the noise in both modes,remove cover then fire up system.
When my fan was on ot its way out the noise was almost painfull to liosten to
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wrote:

I had that with my Suprima 50, but I never did really find out what caused it. Seemed to get better if you bled all the radiators, and now that it has been moved, hasn't happened again (although it currently only works every 3rd day anyway).
Sorry, not much help really...
-- Colin Swan
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