Well I've been under the car and tried to see if anything visible is failing and nothing looks out of sorts. Since any Mercedes mechanic in Dallas charges more than a doctor saving someone's life, I'm going to replace all the rubber on the frame floor bearing (about $400 in parts) and see if that's it. Not the most scientific method but anyone who has ever looked at this car and given me an estimate to do anything (and I mean ANYTHING) comes in around $1500 minimum. If the new rubber on the frame floor bearing doesn't fix it, I'll have to move on to plan "B", whatever that might be.