humming from back passenger side
 

My friend and I noticed a strange hum or something of that nature coming from the passenger's side's rear. All gauges are reading fine, power to all systems is optimal and the car starts faster then any car I'm yet to own so far. What could possibly be making the humming sound? The fuel pump? exhaust? I dunno. All I know is that the volume decreases with the RPMs and increases with them as well.

Check your rear drum. And the hub/bearing assembly on that side.

Will do in the morning

Jack up the left rear, and put your hands at 12 and 6 o'clock. Try to wiggle the wheel in and out, top to bottom. See if you get any play.

If you have to remove the little retainer clips from the lugs to remove the drum, just throw them away.

And make sure you torque the wheels back to 100 ft/lbs when you're done. Let us know what you find.

Does the sound decrease and increase with RPM or with VEHICLE SPEED?

It doesn't do it in neutral so testing that safely with what I have access to is either unsafe or impossible

Just listen and drive. Put the car in neutral while driving 45 and then rev it. Does it change?