What is the most common cause of steering wheel vibration? My wheel only vibrates when going 50 plus mph. It is not a severe vibration by any means, but it is enough to make me want to fix it. My car just got new tires, and alignment. It does not vibrate any more or excessively when braking either. What about the steering damper? I know it's hard to tell, but what is COMMON based upon my symptoms? Or is this just pretty much NORMAL?

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2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)