2001 Saab 9-5 has low frequency vibration/hum after new tires installed

I had to replace the OEM tires on my 2001 Saab 9-5 SE after 50K miles. After getting the new Yokahama "v" rated tires I noticed a low frequency vibration/hum that started at around 10 mph and peaked at speeds greater than 50 mph. The "hum" got worse after a few thousand miles so I brought it back to NTB to have it inspected. The tires showed odd wear patterns so NTB suggested that we try Michelin tires. This did not fix the problem. Has anyone had a similar experience? How did the problem get resolved? The car did not have this unpleasant noise until the new tires were installed.