My Range Rover has recently started to making a "squelching noise - like air being released from a plastic bag" when going over bumps on the left-hand side of the vehicle. I assume it has something to do with the air suspension. It disappears when someone sits in the front passenger seat. The noise appears to come from inside the cabin at the rear. I have checked for plastic bags or anything else that might cause the sound but have found nothing.

Any suggestions?