The problem of excessive vibration in centrifugal oil pumps

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Centrifugal pumps are required to operate in Zone B, meaning that the vibration speed should be less than 4.3 mm/s. However, in practice, especially with older oil pumps that have been in use for over 10 years, the vibration levels tend to be higher; our pumps usually end up in Zone C. The bearings, coupling gears, anchor bolts, alignment, and motor were all checked and found to be in good condition; even the clearance of the mouth rings and the way the pipelines are suspended were inspected. However, the vibration level remains at around Zone B, and it’s not possible to reduce it further (the operating conditions are specified by the process requirements, and generally no adjustments are made). There is no cavitation or evacuation; the motor current is normal. No spectrum analysis equipment is available at present: L). I wonder if other sailors have encountered this issue; what could be the cause? How was that solved again?