Its quite a loud noise - you can hear it inside with all the doors & windows shut. Would a fuel pump make such a loud noise?

Hi there, I've got a 1999 V reg disco TD5 - 7 seater. In the past few days I've noticed that there is a noise coming from both sides of the rear axle that sounds like an air compressor running. When the ignition is on (but engine not started) the sound runs for a minute or two then stops. But...