True, it doesn't explain why now.
I don't have high hopes for my guess either, since coolant also has lubrication properties, but it's worked in the past, it's easy to try, and I believe it's harmless to try.
I'm assuming you used fresh coolant after you replaced the pump, the coolant is full to the top of the filler neck, the correct ratio, that you let any air bubbles bleed out, and that there are no bubbles going into the expansion tank.