I have recently taken delivery of an ex-demonstrator E280 CDI. As soon as I start the engine I get a sort of "pumping" noise for a few seconds which I was told was normal and was associated with the braking system. Fair enough. but I am now getting the same thing intermittently every mile or so which is quite irritating and somewhat disconcerting. It almost as though the car is having to "pump up" the brake fluid pressure to compensate for some shortfall. Is this normal?