Bad Clutch
Definitely a bad clutch. The clutch on my early 80's FilterFlo is going out too. Makes this loud grating noise when the motor starts and makes a loud growling noise while it's running.
When I replaced the motor this past weekend it turned out that the clutch is all worn out. Bearings are shot and shoes are pretty worn, so I too will be replacing mine. I have a 2-speed, so it uses the same clutch yours does.