Hi Alan ----
Somewhere back in the archives of the Vibe Institute - I believe in early eighties - there was a wondeful article published in the Vibe Institute Journal entitled "Vane Passing tones in centrifugal pumps" extremely well written paper, that addresses what is really a pretty complex subject.
As pointed out by others, there many possible causes of excessive vane pass vibe in a centrifugal pump, and all should be evaluated using whatever info and data is available.....however in my experience, by far the most common cause of excessive vane pass is improper location of the impeller (this is especially true when dealing with shrouded impellers) relative to the diffuser in the casing. This mislocation can be axial OR radial. If the hydraulic data checks out, then such geometry errors are about the only other choice. Wear in the diffuser and/or impeller vanes should not be ruled out either. Feel free to contact me if you want to discuss further. I think you have my contact info.