it feels like the body hits the frame on bumps &at high speeds why

Hello, Whether you have the 4Matic, rear wheel drive only, gas, air, or hydraulic suspension, you are likely hearing one of three things: A bad ball joint, a bad shock, or a bad body-frame bushing. All of these will create that type of sound, and the body-frame bushings actually allow the body to hit the frame when they are bad. Have it inspected, and you will quickly learn of the issue.