is the sound coming from the front or rear of the car? if its the front then its most likely your front bearing. if its from the rear, i've had an issue with my fuel pump where the car would hum while accelerating or even coasting. audi issued a recall for the fuel pump a few years ago so that's something worth considering.

 

I've ordered a few things through AutoPartsway.ca, they're based out of TO.

Here's the bearings you asked for...
1999 Audi A4 Quattro 1.8 L4 - Wheel Bearing Kit - BUY 1999 Audi A4 Quattro 1.8 L4 - Wheel Bearing Kit

BTW, I had that same problem with my B7, however I was fortunate that it was still under the warranty, both front bearings replaced by the stealers.

good luck...

 

cheap parts? these are german made cars.. they aren't made cheap for a reason.

 

so I paid $300 plus taxes for the job...it was a front passenger side bearing...thanks to those who offered suggestions and diagnosis.

 

i got 2 wheel bearings off autopartsway.ca for 138 with taxes included... and will be replacing them myself