Does the boiler make a deep rumbling noise? This happened to me once before, I had to get a new Fan Diaphragm, or what Worcester boilers call a 'Bearing Plate'. Make sure to change it soon, because the initial onset of the rumbling and shaking can crack the solder joint.
Similar to what happened to me once before, I was using the Ideal combi boiler, but switched it off before draining the system.
Worked out all OK, no further problems.