There are lots of rubber bits under there that can go bad over time:
sway bar links / bushings
tie rod ends
upper/lower control arm bushings
idler arm bushings
The best thing to do is get under the car with a flashlight and a big screw driver and start poking, prying, and wiggling things and see if you run into something obvious. Most things should be a relatively easy DIY to replace but you'll probably want to spring for a proper 4-wheel alignment once you're done.
Also, it might be worth just stopping by a dealer and asking them to diagnose it for you. I recently did that with my S420 (trying to solve the classic 45mph shimmy). I told them that I had surpassed my ability to diagnose the problem and needed help. They put the car up on the rack, did an inspection, took it for a test drive, and gave me a list or recommended steps to take. They had the car for over an hour and did it all at no charge!
They even washed it for me too.
I don't know if this is standard procedure or not, but it was a pleasant surprise for sure.