I don't know if "fixed" is an accurate word, but still I'm feeling proud of myself anyways. My 15 year old vent hood has been running loud for a few months, sometimes making weird noises. I got through the holiday production season praying it wouldn't die in the middle of a firing.
I was just about to order a new one today, but thought there was one thing I needed to try, before throwing out a big piece of equipment and spending $500. I needed to try vacuuming out the fan, in case the problem was just dust buildup. I took off the duct and looked inside. I didn't see much dust, but I saw a lot of rust on the fan blades. I vacuumed as much as I could. When I turned it on again, it coughed out a bunch of dust then started running quieter. Then I got really inspired and sprayed some short bursts of WD40 up into the motor. Now it sounds normal again.
Based on the rust, this is probably not a permanent solution. But I don't have to buy a new vent TODAY.