I found the culprit!
Back at the food bank, I was able to remove the bottom plate and perform an inspection. It turns out this issue had nothing to do with the motor or brush roll!
What really happened was a tiny rock got stuck in one of the little crevices of the brush roll in between the bristles. It was grinding against one of the rug strips on the bottom plate. That's what was making the vacuum sound like a chainsaw!
So I dug out the rock, and now the vacuum works like new again! How crazy is that?! This really saved the owners from a trip to the vacuum shop! The food bank owners thanked me for my good deed.
Below is a pic of the rock still in the vacuum. The second pic shows me holding the rock in my gloved hand.
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