My Mazda 3 makes a very loud bumping noise but only at certin speeds (40's & 60's)?

I have had my tires balanced and rotated but nothing changed The noise sounds like I have a flat tire or like I am running over the small bumps they put between lanes so you know you are drifting. I dont want to pay to have random parts replaced because no one can tell me what it is please help!
 
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