What causes the car to sound like a lawnmower/race car once you hit 50+ mph?

What is your question?

I bought my car in August of 2008. It has ran like a dream up until about 2 months ago. When I drive over 50+ mph the car sounds like a race car like it's taking all it has to run. Once I slow down to under 50 it purrs like it's supposed to. I've had the front end aligned, tires balanced and rotated, one set of brake pads replaced, but I still don't what causing the sound. It sounds like it's coming from the tires, but I replaced one of them and it's still a problem. I'm frustrated with it. I have to drive to work with the music turned all the way up to drown out the sound. The front end doesn't shake or anything it just sounds really funny and I know it's something because my car didn't start making this sound until a few months ago. Thanks for your help in advance!