It doesn't seem likely the motor pole pass frequencies would show up on pump bearings but in case they exist on both motor and pump (transmitted from motor), you can certainly check whether 2.5hz is your pole pass frequency if you know the exact speed (slightly below 3000 cpm). 2.5hz = 150cpm would be pole pass frequency if running speed were approx 2925rpm.
2nd guess as you have appeared to rule out is fluctuation related to the fluid system... valve or load changing at 2.5hz.
If you rule out the above you have definitely ruled out all the likely candidates. The only other things I can even take a wild guess would be oscillation in the power system (possibly including vfd if present) that might cause torque oscillation at 150cpm.
And if I brainstorm what might possbily oscillate at 2.5hz to cause sidebanding I can guess resonance of very very flimsy structure (spring mounted machine or extremely flimsy cantilever support). Actually I did have spring mounted machine that had the spring resonant frequency (200cpm) as sidebands to 1x. It didn't fit any of the charts but I postulated possibly as the machine slowly wobbled on it's base it was changing unbalance or misalignment response (even though both motor and fan were on same base). Also possibly torsional resonance of flexible coupling... never seen it but seems possible.
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