2001 Chevy S-10 knocking sound

I have a knocking sound coming from my S10. It is a weird sound that only occurs while I am driving. The sound starts at about 20 miles per hour and is constant until you come to a stop. I have replaced the front wheel bearings. Left side at about 45.000 and the right side at 65000. A friend thought it might be a universal joint. Another friend said they thought it could be tires. I just had the tires rotated and the front end lubed and they did not indicate any problems. What might it be and how do I go about troubleshooting the problem?