Another noise question

I drive a 2001 Sable wagon. A couple of weeks ago, after changing the rear shocks, I started hearing what sounded like road noise coming from what I believe is the front of the car. I is there only when driving. The engine sounds normal. Good acceleration. I though it might be the transmission, but it shifts without problems. I cannot locate where this noise is coming from. (yes, I have new tires!). Any clues??