Your M 5 doesn't have air suspension, the coil retainers that hold the coil springs in place sometimes slide past the retainers causing the spring to fall lower from the set point, have the rear shock and spring inspected for wear or broken retainers.
Vic
RepairPal Certified shops can diagnose and repair your 2001 BMW M5. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.