Turns out, I miscommunicated with my customer. After checking the brake pads, which were like new, I put in a load of laundry and tried again to replicate the squeak noise. No such noise - the brakes worked perfectly, but the spin speed seemed excessively fast. So fast it was out of my comfort zone.
That's when the customer came downstairs and said "There - that's the noise I was talking about. That tornado sound."
The basket was spinning so fast it did have the sound of an approaching windstorm. I tried it again on Delicate and it slowed right down to approximate normal. All other settings, it would spin like balls afire.
I don't know what would cause that. This is a 2-speed motor but without a speed control selector. I can test the speed switch in the console and ohm out the motor, that's all I know to do right offhand.