Could be either the motor bearings or the pump seal. My guess, and that is only what it is, is if it is the seal then running it dry and then with water will change the noise. Motor bearings would not be influenced by the presence of water. As for just replacing the whole pump and keeping it for a spare...I find an old pump very handy at times. When you redoo the plumbing definitely include unions. As for the PVC slip joint...never saw one. Mine squeals for a couple minutes when really cold, freezing or below, but then quiets down. Been doing that for years.
Al