When I start my 2011 Range Rover Sport HSE, a hear a deep rumble noise which sounds like it's coming from the rear of the vehicle, it'll make this sound for around 20 seconds and then suddenly disappear, I have no active warnings on the vehicle when this noise is present, the car is in normal ride height. After the noise has gone, I could drive around for around 20 or so mins, and after I've stopped, e.g: been sitting in traffic for a while, the rumble noise will return for typically a shorter period of time, around 10 seconds and then disappear again, I'd assume it does it whilst driving, but it can't be heard easily with the vehicle in motion.

My guess, is that this is potentially the air compressor activating to simply keep the air tank topped up, it's a 2011, with 96,000 miles on it.