Creaking noise from back

[rayxi]

I've noticed a creaking noise from the back of the X5 when starting from a dead stop and sometimes when coming to a dead stop. I hear it when I press the accelerator, not when I let my foot off the brake. The noise is there even if I accelerate gently.

If I gun it from a slow roll I don't hear it, I only hear it from a dead stop. As long as I'm already moving, I never hear it, regardless of how hard I accelerate or decelerate.

Any ideas as to the cause? Bushings? Levelling air pump?