I've got a strange problem with my combi...

When hot water is drawn, the front panel of the boiler drums violently for 1
second every 8 seconds or so - this continues for 5 or 6 times and then
stops. There is no drumming when the heating is on, unless hot water is also
drawn.

I've taken the outer cover off, and it appears to be the front of the
combustion chamber resonating wildly. It doesn't seem to be affected by the
pump rate, temperature settings, or the hot water flow rate. The radiators
all work, and haven't needed bleeding - pressure also appears to be holding
nicely.

On a couple of occasions, there has been dribble of what looks like rusty
water from the end of the horizontal flu. I'm not sure if this is relevant,
but I mention it for completeness.

Any ideas?