jbeditor

I have a 2001 CLK55 with 255/35/18 size tires in the rear and 225/40/18 tires in the front. I had new BF Goodrich tires put on approximately 4 months ago in the rear and about 2 months ago in the front, so all tires were relatively new. For some reason, I have been hearing a dull whine with a slight reverberation sound that becomes audible at about 20-30 mph and above. I had my tire store take each wheel off and examine them and they found that 2 of my rims had flat spots, which I subsequently had repaired. So now all wheels have been verified to be "true", no oddities were found in the tires either, but the sound persisted as if nothing had been done. The tire store inspected all areas of the suspension as well as the bearings but found nothing. As a last resort, we replaced all the BF Goodrich tires with Michelin Pilot Sports and there seems to have been an ever so slight abatement of the sound, but not much. I don't know what else to tell the tire store, they are willing to investigate, but are out of ideas too. Anyone have any advice?