As D ave and Tony have said its a case of replacing fan assembly.
This is a very easy task for you to do.
Fan assembly part number, 8 717 204 325 0.
Dave, this was discontinued in 2003, though agree a registered installer warranty may still stand . By the way what was you and Tony doing in Leeds?. Wish I had Know you were there had an R`n`R day Friday
This is in fact a very common problem with this boiler.
There is a non return valve made of thin rubber just before the fan impeller.
It has probably come to bits and the rubber flaps caught up in the fan impeller, causing your vibration.
Just to let you know, that the entire fan assembly is a gas carrying component, as the gas is entrained into the fan along with the combustion air.
So you really should get a gas engineer to fix it.
I had this once but went round the next day with a new fan and it had stopped and hasn't done it since. I hope it is a common problem as the fan cost me about 300 quid and ended up just sat in the van.
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