There are lots of bushings that can cause this problem. I had the same problem with my front end. The castor joint bushings were really worn, and infacct the castor arm joint has rusted to the point that new bushing alone wouldn't work. I had to have the castor arm removed and sandblasted/glassbeaded. Then new castor joint kit (bushings and such). It made such a difference, both in ride quality, better steering and tighter ride, also no more clunking. Have mechanic check it out. My mechanic figured out my problem so fast he didn't charge me for the 10 minutes the car was on the lift. Two weeks later when he fixed it the cost was $900. Pricey, but most of it was the labour and machine shop costs (new castor arms are like 900 bucks a piece). Anyways I am happy, he knew by looking and poking around what was wrond right away). My suggestion is to have it looked at.
1995 E420 - retired
1986 420SEL - retired