2000 Chrysler Cirrus Humming Noise

Going 35 miles an hour or more my cirrus makes an obvious humming sound. I cannot pinpoint where it comes from. It seems to be coming from the front somewhere. When going about 50 miles per hour, I give it a tug to the left and this makes it louder then I give it a tug to the right and the this makes it more quiet. I brought it to a repair shop and they replaced the right front wheel bearing but this did not help. I brought it back and they replaced it again in case the new one was defective but to no avail. I still I have that agrivating hum under my car. They checked out the whole front end and could not find anything else wrong. The only things that I can think of are, the left front wheel bearing, or a rear wheel bearing echoing to the front, or the transaxle carrier bearings. Can you give me a sure method of weeding out this problem? Thanks. Dennis