My car make a humming noise between 35 - 80 mph?

I got my driving belts changed and did an oil change the same day. Since then the car has been making a humming noise which is very annoying and I have been avoiding it on the highways. I took it to the dealer and he said it's coming from the tires. I don't believe it is true as it just started right after I got the belts replaced, but they insist I spend 800 bucks to get new tires. The car I have is a 2004 Infiniti G35x. Any suggestions on what the noise could be?
 
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