Can't pin-point my problem

I was wondering if someone could shine some new light on what may be wrong with my 99 Grand Am.
I just got back from a long road trip and about half way back I heard this loud noise coming from under the driver side floor. It sounds as though there is a small engine airplane under my car. At first I thought it was my exhaust, but I couldn't find anything wrong there. When the car is idleing, I can't hear it. I can only hear the noise at 15 mph or faster. It sounds as though something is grinding. Once I hit 15 mph the noise is constant doesn't get louder or softer when I stay on the gas. When I let off the gas is becomes like a winding sound that gets slower and slower until I hit the gas again. Then it's constant again. When I turn either way the noise stays the same doesn't get worse, doesn't get better. Also, I can feel a slight vibration on the floorboard on the driverside that isn't usual car vibration. I took my car to a dealer to get an idea of what the problem may be. They said that the hubs and bearings on both sides are bad, but they said the left hub and bearing were worse and that is what was making the noise. So, to save money I decided to change the left hub and bearing myself. I did, but the grinding noise is still there. I can't figure out what it could be. Please help.
Thanks
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your brake pads may be fine but you may have warped rotors they can make a grinding noise or inside brake pads , there hard to see and easy to assume that they are fine when the outside pads are still thick. extra ideas.

check the right hub and bearing..when i had to replace mine it came out in 2 pieces..only supposed to be 1 unit. so i would check that out

my 99 had that howl sound and it was the right front bearing.

like they said, it could be warped rotors. If you apply a consistant amount of pressure on ur brake pedal and your brakes seem to grip, ungrip, grip, ungrip..almost like your brakes are only grabbing certain points or even like a slow motion ABS, then its more than likely warped rotors but if applying consistant brake pressure results in a smooth stop then its something other than warped rotors.

hmm..hope that bunch of gibberish makes sense

Sounds like the catalytic converter came apart it would sound just about how you said.

could be an exhaust heat shield came loose, and only makes noise under motion as the air passing by makes it flap.