With the motor having been rebuilt, the pump and process have yet to be ruled out. It appears the vibration is related to process.
Has this issue been present for years or just showed up, your charts are showing a short history of the machine.
How is suction side designed? How deep, approximate pit dimensions, fluid temperature, is the surface of the water smooth or disturbed and swirling, has temperature of liquid being pumped change?
Do you hear any noise change when the pressure drops on the pump?
Have seen suction vortexing, worn "bell' on bottom of pump causing flow changes, pump not submerged enough in liquid, poor mechanical fits between pump base/adapters/pump head, pipe strain, lack of piping support and nearby machinery operating all create low frequency vibration. Have you/can you vary the liquid level on suction side of pump?
If we understand correctly the NDE is the top of the motor which has the 7.5 hz vibration, assume the vibration at 7.5 hz has lower amplitudes on rest of pump? Can you provide FFT/spectrums of NDE x,y,z DE x,y.z and any pump vibration readings.
As others have stated, a series of bump tests should be done on motor and pump to verify a natural frequency is not close to 7.5 hz.
Have you done any running soft foot checks on the motor to pump interface, pump baseplate, adapters?