Some of these problems are the ones that leave you with your mouth open. I doubt that if will be vibration, since it would have been an original problem start with. I would recommend doing something similar to what the last poster described, and at the end, you will not have any explanation to...
I agree with the previous poster as this is a cavitation problem; however, I will also take a look at how long did it took for the impellers to fail, material of construction and solids in the liquid. I think is more of a mixture of problems.
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