[thstone]

Diagnosing a "hum" (or possibly two different "hum's") over the internet is really hard - especially when it only occurs when you increase throttle (which increases the background sound level).

I listened to the recording, but to be honest, its hard for me to pick out the sound of concern. If you can get a clearer recording and include some annotation as to exactly when in the recording the hum occurs, it will give folks a better chance of recognizing it and providing some useful feedback.