Thanks for the reply,

Sorry, off the top of my head, I don't know what kind of engine mount
it is. I shall ask the aircraft's owner.

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

What I should have done at the time was vary the RPM more and see how
the 3.5 order vibration magnitude changed. That surely would have
indicated whether it was a resonance at a particular RPM.

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 and
acceleration means force!

Mark