Hmm, I don't see the CMCON and ANSEL in your program.

There's a R-M-W (Read-Modify-Write) problem when OUTPUTting to a pin set to Analog. It adds a high frequency component to the PWM signal.

I don't know if that's what is causing the growling or not.

6.5V at full speed? That doesn't sound right. The current is getting limited somewhere. Maybe if you increased the value of R4 ????
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