Noticed recently that there is a noise coming from behind the drivers footwell area when the ACC brakes. It sounds like something taking in air where it shouldn't be, like a pipe with flowing water taking in air - a big ole slurping sound.

This doesn't happen when braking without the ACC (ie manual), and full braking power is always available even with the noise present.

I could start investigating if I understand the components that would actuate braking when the ACC is switched on? 2011 Vogue SE 4.4 TDV8

The ABS pump runs to operate the brakes in the same way that traction control works, opening and closing valves in the ABS modulator as required to apply appropriate braking force.