How lightly can you strike a bell so that you cannot hear it? Your situation reminds me of a 2 pole Siemens motor on sleeve bearings that believe it or not had an outboard end bell that was perfectly resonant to running speed. Fortunately the manufacture left some sizable threaded holes on the end plate to which we threaded some redi-rods. We then inserted some substantial 5/8" plates to de-tune the end bell. The rotor had been shop balance to 0.1 mill at an unknown speed, at least that's what the balance report claimed.

Yvan,

Well they do call it an end bell

How lightly can you strike a bell so that you cannot hear it? Very good analogy.