Don't rule out a separating tread or defect in the tire itself. I went through this recently in my Honda and it was due to a TINY tread separation in one of the tires. Balancing/aligning/etc did nothing for it and I got a TON of flak over it from Discount Tire not wanting to honor their warranty. They've lost my business forever over it.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]