I have googled around this evening but cannot find any leads to this problem

Boiler is a Ariston EuroCombi SX20MFFICE, very common when iI fitted it
about 7 years ago

Now recently it has had a few problems..
Expansion Vessel lost its pressure, but managed to re-presurise it before
Christmas and it has been OK ever since.
Recently it developed a leak and that was traced to the main flow switch
which I replaced on Friday. At the same time I drained the whole system down
and added CH cleanser as the primary circuit water was somewhat black. The
system has always had inhibitor in.

Now the new problem is that when drawing off hot water (DHW), the boiler is
noisier than it used to be and the noise I can only describe as a rumbling
noise like a fan or motor with a knackered bearing, get worse the longer
water is drawn off. But when the hot tap is close the noise turns into a
kind of shuddering noise briefly, but it is quitet load. There is obviuosly
something wrong. Not sure if it is trapped air, but I have had the auto vent
valve open and have tried beeding of the temp sampling point just before the
heat exchanger. The noise is not present when the CH is running, only when
drawing off hot water (dhw)

So ant other ideas from you knowledgeable folk ???

TIA

DeeBee