Clank when accelerating from 2-3k RPM

2002 Freightliner. 140k.

While trying to acclerate from low speed engine seems to settle in just below 2000 rpm and then I hear an audible clank clank and engine climbs to higher RPM. Clank is sometimes solo, double, or triple but accompanys the acceleration and disappears once that happens. Less likely to occur if I stay in first gear until getting over 2000 rpm.

Engine runs fine afterwards.

Any suggestions?

Some 2002s came with a thinner, light duty Flex Plate between engine and transmission that can cause a loud metallic sound when it over-flexes under load . A stronger flex plate was made to replace it on later Sprinters, and can be installed if you don't want this one anymore