Yesterday I installed a 'good' used fuel sender, complete with pre-pump. Before closing everything I plugged and unplugged a few times to verify that the sending unit worked, and that the pre-pump worked. Both worked fine.

However, the main fuel pump now is making a loud high-pitched whinning sound. Does this indicate that the pre-pump now installed is not working as hard as the one just removed with the dead sending unit? (pre-pump on old sender is only about 1 1/2 years old)

Does this mean there is an air lock? ( I drove 300 km today, and no change in the main pump sound.)

Of does this mean the main pump is about ready to pack it in?

Your suggestions gratefully accepted.

Many thanks!