New water pump making whirring/roaring when cold - M103 '88 300SEL

[86560SEL]

We finally had to have a new water pump installed on my mothers 1988 300SEL (M103 engine).

We bought all of the parts and had a mechanic do the labor. I picked the car up, drove it home - the car was already warmed up, I guess from the mechanic running it, checking for leaks. He put on a new water pump, thermostat, belt and drained all of the old coolant and refilled with new Zerex G05 coolant.

Well the car set for a few days, as it isn't not used much. I was going to drive my mother to town and when I was warming it up, I noticed a roaring/whirring noise at certain rpms. We started to leave at it was making a really loud noise, started back home, but then it started to quit. We went on and it never made another sound. I kept a check on the gauge, it was running in the normal range and no leaks.

My question is - is there a break-in period for these? I don't think it should be making any noise. I was also thinking maybe there was air in the cooling system causing it. I don't know.

I am trying to do a little research before calling my mechanic. It was a pretty big job labor-wise and I hate to think it may be a bad NEW water pump and it has to come off again.

Thanks.