1997 Lexus LS400 makes clunking noice in steering when going overe bumps at highway speeds. Already replaced struts and lower ball joints.
1997 Lexus LS400 with clunking noise in steering on 1999 Lexus LS400
Have the lower way bar links checked and confirm the top strut mounts are OK. As Dave said noises are sometimes hard to diagnose but make sure a technician goes for a ride with you so you can point out the noise you are concerned about.
Noise are so hard to diagnose here online. You really need a good tech to take a ride with this vehicle and feel what you are feeling to give you a good idea of what could be causing the noise.