When you hear the noise, did you try using your left foot on the brake lightly to see if the hum goes away? Don't forget to look at the ty rod bushings. If the bearing is starting to go to heaven, you should be able to feel play in the wheel by rocking the wheels by hand.

It's possible that your new pads aren't seated properly yet. A couple of 70 - 0 panic stops in a row will help with that.
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95 854T, 88 780, 88 245, 01 A4 Audi