Here's a weird one.
I recently replaced a bud's '99 water pump in an effort to get rid of a horrendous drive belt noise.
As it turned out, both the belt tensioner and idler were at fault, but, and this is where it gets strange, the pump impeller was starting to wear the pump housing away ! There was actually a wear pattern on the housing from the impeller....
Mind you the pump seal was not leaking, nor the bearing making any unusual noises
I've replaced many a Ford water pumps before, but this one is strange...
BTW, the pump was the original unit with 130k on it
Has anyone else seen this happen?

 

I’ve seen this happen over the years at work with industrial pumps, 8", 16", 24",40" etc, it’s pretty neat to see them on such a large scale, as to why, in industrial pumps the housing is tightly fit around the impeller and gradually decreases in size towards the discharge side, creating a small additional amount of velocity energy in addition to the centrifical energy, and the fluid kind of starts to make "grooves" like a wear pattern if you will :) and gradually wear out the casings reducing the pumps efficiency, but this is with hundreds of millions of barrels through the pumps . It’s pretty cool to see them on the larger pumps :)