Pump Motor Squeal. Causes?

I have a Hayward LX pump and every once in a while the pump will make a loud high pitched shrill sound. It happens when I remove the basket to clean it and then reseal it. As the pressure increases the sound starts and then like magic it goes away after a while (sometimes 15min or 2 hours).
The rear seal has leaked, but doesn't now and the pump functions fine otherwise. Once the noise stops it won't re-occur until I remove the basket cover again. It's on a timer and when it switches on it sounds normal.
I am puzzled. It is a very strange problem.