What's wrong with my car & how serious is it?

Bad gas would produce a knock or rough running, not a hum. Is there smoke coming from the exhaust when it's hot? Water could have gotten in there and you're hearing it bubble as it boils and burns off. Theres no harm in taking it to a mechanic for them to really check it out, it's probably easier for them to diagnose it when it's in front of them.

Good luck!

Not very sure what you could be describing? The only thing i could see happening is maybe you developed an exhaust leak? is this humming coming from the engine bay or from somewhere else? the only way anyone could give you a decent answer would be to have a one on one conversation or give a better description of your problem.

Check your air filter. If water got in there or anything else, it could have gone into the engine. The filter may need changed. If your check engine light came on, you can always stop at an Auto Zone and get the computer read to see what the problem is.

First off, where is the noise MAINLY coming from. Because, there are many different things on a car that can cause something to do this. It could be one of the belts (unless you have a Serpentine belt)... it could be alot of things. don't try to fix it yourself though it may lead to more undesireable things...

It also could be a bearing or a joint got water in it

bad gas wouldn't cause humming.. more likely cause knocking or bad running

got a wheel bearing going out/get to deserted part of the road/straight line and 35 mph and load car by swerving gently and see if noise increases or gets lighter/confirms wheel brg