Welcome to the square-body family, Chet.
Your fan motor is likely to be the source of the growl. If so, it is time to replace it. (With the device you referenced unplugged, you only have the high speed blower circuit connected. High speed doesn't go through the same circuit as the lower speeds.)
By the way, when you remove the old fan motor, inspect it to see whether it is the actual cause of the noise. Check it physically by manually rotating the squirrel cage and noticing any noise, binding, or excessive play in the motor shaft. It should rotate quite freely. If any of the described conditions exist, the motor is the culprit. Also check it in operation. You can do this by connecting it directly to a 12V source. (Be careful, as the motor will rotate quite fast. Don't get a finger caught in the squirrel cage !) ANY stray noise observed at this point is an indication of a motor that is going bad. I say this to emphasize that it is best to inspect and confirm a bad part before simply changing out one part after another until the problem goes away. That may not be what you implied, by "What parts should I start changing out?", but it is the way some people do diagnosis, and I just wanted to throw that idea into the discussion for the benefit of all.
As to why you don't hear the growl on high, the high speed produces more air noise and I suspect that masks some of the growling. I believe that if you replace the blower, you will notice that not only is the growl gone on the lower speeds, but that the motor is operating quieter on high as well.
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