Turbine gearbox vibration

In the oil site, we have a gas turbine with a power turbine with 8700 rpm speed and it has a pump with a speed about 2700 rpm. the pump speed according to the control room computer gives me the percentage of 90%. The pumps that push the oil to the turbine are two boosters. The capacity of each pump is 4000 cubic meters per hour. The recycle valves of the two pumps the return valves to the tank, are closed. only suction and discharge. This gives me, in total, the final capacity of the turbine pumping, which is 6600 cubic meters per hour. The problem we faced was the vibration in the gearbox of the engagement between the pump and the turbine shaft. The question is, if we reduce the speed of the turbine pump and make it approximately 2600 rpm, and turn on a third booster, but the recycle valve for that booster is fully open, the capacities of the two pumps, two of them are full power and the third is half, does this reduce the vibration, since the pipe is completely filled with oil and there is no vibration, or does it reduce the vibration? Note that there is a whistling sound in the pipe, which you think means that the pipe is not full. Fully oiled, noting that the suction pressure of the turbine, which is the sum of the boosters, is 8.5 bar, and the discharge pressure of the turbine is 42 bar.