Humming noise above 20MPH
 

My 2000 CE has started making a wierd humming noise at around 20 mph and continues on up. It doesn't seem to get any louder the faster you go. I had the brakes replaced and all fuids checked,topped off, to include the differential. It only does it while moving. I can sit in Nuetral and Park and rev the engine up and I don't get a noise. My mechanic put the car on the lift and ran it in gear and still couldn't figure it out. Any ideas!! Timing belt, seals, etc have all been replaced, twice now actually. The car has a 4cyl and just passed 200K miles. It doesn't seem to be a bearing and I had the tires balanced. Thanks for any help on this.