I tried the "hot hub" test. The front passenger side hub was slightly warm while all the others were cool. I forgot to do the swerving test but did do the braking test. The hum stoped when I braked but started again immediately after letting up on the brakes. I'll focus on the front passenger wheel. On the Mercedes Club forum one responder said that he had a similar situation as mine and had the bearings repacked at a Mercedes dealer and the hum was still present. It turned out to be a brake caliper touching the disk.