I have a 2013 dodge charger r/t max awd. I noticed that the rpm gauge keeps bouncing between 1500 and 2000 rpms while driving. The other thing I noticed is that there is a noise coming from the back end that almost sounds like its having difficulty in the differential. Any ideas on what could cause this or how to fix it? I've read that the rpms could be caused by a vacuum leak somewhere and the rear end could have some sort of software update needing done? Any help will be appreciated.