The shocks are secured to the vehicle (or chassis) by using 2 rubber grommets on the top shock mount. The grommets get soft, primarily the one between the shock and body, and when you hit a decent size bump the shock simply hits the body. In my years of service I do not recall running into upper shock mounts that were loose. Perhaps when the vehicle was relatively new (<10K miles) or the shock was serviced within a short amount of time (< 3 months) prior to the complaint. If the grommets are the cause of the knocking noise I might encourage a vehicle owner to replace all 4.

I am at a loss why it would take 6.5 hours to replace two accumulators so for good measure I can only suggest double checking the following items for tightness and or wear.

Sway bar links
Multi-link bushings
Shock mounts (top and bottom)
Outer control arm bushing (hub to control arm joint)
Sub-frame mounts

Good luck