alignment problem or something else?
 

I have a very slight road vibration at highway speeds that sounds like a tone that changes in pitch over a slow cycle of about 2-3 seconds, like a resonance. It's definitely coming from the tires, seems to be louder on the passenger side. Tires are brand new snows, mounted and balanced on brand new rims by TireRack. Car is 1999 E300 TD. I noticed it too with my summer tires. Was waiting to see if I would notice it with brand new tires, and now I do. I noticed some cupping on summer tires, but they are worn pretty bad and will need to be replaced come spring. Car tracks perfectly, does not pull to side, and ride quality is great in every other respect. But I've had the car over 2 years and not done an alignment, should I try that?

good vibration
 

Try driving on different road surfaces. Concrete vs asphalt.......
Tires do make some road noise ........
You may want to check your alignment I replaced my lower control arms at 80000 mi. ...as you know these cars require much TLC......