No argument Frosty - it's cavitation.Here is a quote from a document on water pump cavitation....it's a well know phenomenon.

''Cavitation in a centrifugal pump has a
significant effect on pump performance. Cavitation degrades the
performance of a pump, resulting in a fluctuating flow rate and
discharge pressure. Cavitation can also be destructive to pumps
internal components. When a pump cavitates, vapor bubbles form
in the low pressure region directly behind the rotating
impeller vanes. These vapor bubbles then move toward the
oncoming impeller vane, where they collapse and cause a
physical shock to the leading edge of the impeller vane. This
physical shock creates small pits on the leading edge of the
impeller vane. Each individual pit is microscopic in size, but
the cumulative effect of millions of these pits formed over a
period of hours or days can literally destroy a pump impeller.
Cavitation can also cause excessive pump vibration, which could
damage pump bearings, wearing rings, and seals.''