rayzer

I have a 2004 Silveraldo 2500HD 6.0L 4X4. The last couple of days I have noticed that while driving over 45mph that if I am just maintaining speed or coasting that you start to hear a rythmatic knocking comming from the front end I believe. Dosent happen all the time but is noticable. Can't feel it through the steering wheel but it stops if I speed up or lightly apply the brakes, can't feel it in the brake pedal. Checked all lug nuts, there tight. Wheel bearings maybe? How much to replace?