When I put a blind filter into my two year old Quick Mill Alexia and turn on brew lever, I get a pulsing sound. The pressure gauge oscillates between 10 bars and 9.5. Thinking I might need to replace the pump?

sounds fine if it's still hitting 9.5 - 10 bars.
is there a PID in there?
Might well be the PID Switching the Heating element on and off that's fluctuating the power.
Does it happen both when the Heater is on and when it is off?