Newbie with queries

Hello there. I've recently bought a Discovery TD5 XS as I needed a proper 4x4 for towing and launching a boat. I have a couple of issues the first being a notchy transfer gear for changing to high and low gears. Could it require oil? Also, when playing the stereo, every now and then a warm sub bass kicks in, before buggering off again. I've checked the plugs are secure on the amp under the passenger seat and all seems ok. Is this something anyone had come across before? Any advice would be greatly appreciated.