Checking the transducer mounting is a good idea even if it only
removes another unknown from the equation -

Maybe your xdcr mount is resonant at the 3.5 harmonic? Change the RPM
& see what the measurments show.

What's nagging at me is the magnitude of the 3.5 order vibration. 0.5
IPS equates to quite a lot of acceleration at that frequency

Its still only 1/4 g approx. Not that much for a piece of cheap
machinery.......!