Strange noise at start up X350 Diesel

[PacificBlue]

Can anyone help? I own an '06 X350 TDVi and at start up from cold I experience, what can only be described as, a 'thrubbing' noise coming from behind, or in, the pax dash board. The noise is not in synch. with the engine and only lasts about 5 minutes. It starts at a low frequency gradually increasing until it stabilizes after about 5 seconds. It is not audible outside. I thought at first it might be a noisy suspension compressor but there is no noise from the front L/H bumper and it would bring a light on if it ran for more than 45 seconds. Any ideas?