I have a 2012 Hybrid Cadillac Escalade. A few seconds after turning it on there is this loud noise that comes from the rear. Lasts maybe 4 seconds. Any idea what this could be?

 

usually it's the air pump for the air bag level. from what I read on every startup a short time after it runs the pump for 4sec as a self diagnostic for the system. if you hear it running much more often than that, it can mean you have an air leak in the system. I read it's pretty common but mine hasn't given me any trouble with 140k on it.