Water pump knock... how/why?

After doing some searching I was unable to find what exactly would cause a water pump to create a knocking sound and was wondering of someone here could offer some insight as to how or why a water pump for a 350 (5.7l) would knock. Anyone know or?

A little back story:

The other day I had to change the water pump on my '96 k2500 (w/ 5.7l). Bought a brand new pump and put it on, refilled the coolant. No odd noises. Heater blew out very hot air, too. Drove for about 20 miles then parked it for about 6 hours. the radiator was still full and no signs of leaks - everything looked normal.

Within 10 seconds of starting the truck there was a rapid knocking... like numerous valves were suddenly out of adjustment or rod bearings crapped out. Shut it off and rechecked everything started it again. The oil pressure was steady at about 55psi. shut it off again.

With the serpentine belt off, oil pressure was good and there was no knocking. Put the belt on and the knocking returned. I also tried it with the fan removed and the belt on - still knocking.

Returned that water pump and got another and so far no problems.
But I still wonder, if it was a bearing or maybe some impeller tolerance was off or what exactly in a water pump could cause a rapid knocking sound. Anyone ever had this happen before and found the culprit?

After doing some searching I was unable to find what exactly would cause a water pump to create a knocking sound and was wondering of someone here could offer some insight as to how or why a water pump for a 350 (5.7l) would knock. Anyone know or?

A little back story:

The other day I had to change the water pump on my '96 k2500 (w/ 5.7l). Bought a brand new pump and put it on, refilled the coolant. No odd noises. Heater blew out very hot air, too. Drove for about 20 miles then parked it for about 6 hours. the radiator was still full and no signs of leaks - everything looked normal.

Within 10 seconds of starting the truck there was a rapid knocking... like numerous valves were suddenly out of adjustment or rod bearings crapped out. Shut it off and rechecked everything started it again. The oil pressure was steady at about 55psi. shut it off again.

With the serpentine belt off, oil pressure was good and there was no knocking. Put the belt on and the knocking returned. I also tried it with the fan removed and the belt on - still knocking.

Returned that water pump and got another and so far no problems.
But I still wonder, if it was a bearing or maybe some impeller tolerance was off or what exactly in a water pump could cause a rapid knocking sound. Anyone ever had this happen before and found the culprit?

many years ago on a 1973 350 engine the pump started to weep and then the bearings made a tap sound not too low but the sound was something I never heard again.

most products today are not tested so it is a problem for us in making repairs , on occation several times to complete the repair.

where did you purchase the pump ?

chinese bearings most likely ..the hub bearings from china stay away !

It was from Carquest. Never had any problems with their parts in the past 10 years.. *shrug*

edit:
did some searching and if the secondary p/n is from the manufacturer then it looks like GMB makes the pumps for carquest.

It was from Carquest. Never had any problems with their parts in the past 10 years.. *shrug*

edit:
did some searching and if the secondary p/n is from the manufacturer then it looks like GMB makes the pumps for carquest.

carquest parts are good but more parts are from china. quality is going down.