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Bottom line up front - pulley cracked around the arbor, so pulley was wobbling and making noise at idle. Off idle it was okay, but I thought the water pump proper had failed. Changed it on a bank drive through for shade. Fortunately I had a new pump with me. Something to watch out for, but maybe only on the high mileage C5. I know a lot of you want to keep the mileage down and save it fir the next guy.
I have a 2000 base 6spd coupe with 185,000 miles. Quite possibly the stock water pump started making noise on a 4000 mile road trip, which included a track weekend at Ozarks International. I bought the car with 174,000 miles on it in Arizona last year. I bought a water pump, just in case, with the intent in changing it while changing the harmonic balancer, but didn't end up doing it. My recommendation is do it before you are doing it on a trip!
Hope this helps! Notice crack around the pulley arbor
Water pump pulley failure
