Humming from the back of the car could be caused by some cupping of the rear tires. I had this problem and was wearing the rear tires badly in the inside edge. What he said about the front tires sounds like BS. How does the tread look on all four tires? does the tread depth look even accross the tires? Feel the edge of the tires and see if you detect any flat spots. There are wear bars between the treads, are you worn down to those? IF you have any weird issues you may want to get an aligment before hurting a new set of tires. A really good alignment shop can look at your tires and set your car up for maximum tire life instead of maximum performance.

If you buy tires and are not worried about all out performance, look for a tire with a high tread life rating and a mileage guarantee. Another option which is what I did, it to switch to smaller wheels like 18's all around or 17's in the front and 18's in the rear. I was able to get wheels an tires for the cost of just stock size tires. When you go to smaller wheels you open up the possibilities to many more brands of tires that are much cheaper. when you go to smaller wheels you are also going to have a little higher sidewall which will improve the ride. Also you will be reducing unsprung weight which also helps the ride.