The rack is the final actuator link inside the inj pump. Similar to a rack and pinion. All of the pump barrels are geared to the rack so that through one action all delivery chambers change accordingly by rotation.
The pin acts as a dampener, at idle delivery, against the rack. It keeps engine vibrations from influencing delivery amount and causing an oscillation in engine speed.
Your problem sounds more like mounts as Mike said.
Bosch Master, ASE Master, L1
33 years MB technician