Welcome to the forum!
First, I would suggest that you take it back to where you got it and ask
them to fix EVERYTHING that you mentioned above.
Here are some possible causes of your issues.
Humming noise: This one I'm not too sure about. Can you tell where the noise is coming from? Noises are hard to diagnose over the Internet! I would suggest that you start the van and "snoop around" while the noise is happening and try to locate it more precisely.
Gas Gauge Fluctuation: Often the sensor (in the gas tank) gets dirty over time. When this happens, the gauge can jump around quite a bit. Some folks have reported that injector cleaner (or similar) in a few tanks of gas "make it better", but I believe the proper fix is to replace the failing part.
Flickering Lights: I had this problem on my van. There is a TSB (Tech Service Bulletin) that says that the ground screw in the generator (alternator) is/was not making a really good connection. Check out the following page for more information:
Flickering Lights
Low Coolant Indicator: Make sure you check the coolant level at the radiator neck, not the coolant recovery tank (when the engine is cold). The sensor for the coolant level in the radiator on the passenger side.
Door Ajar Indicator: I would suspect dirty connections at the sliding doors. Check the pins & pads on each sliding door and clean them up with steel wool and/or alcohol.
That should get you started. Let us know what you (or the dealer) finds.
Take Care,