Yeah, I have two of them... a 9100s and 9100t and both do that. One on the steering and one on the throttle/brake.
Listening to your video your 9100 acts just like mine. Buzzing at the ever so slight force against it. You can take your finger and slightly touch the servo arm to make that noise happen. When your turning your wheels sometimes it's quiet at neutral and sometimes it makes a buzz. Just means there is very slight pressure remaining on the arm.
If there is absolutely no resistance, and I mean NONE, it will go quiet just as
your video shows.
I think your servo is fine.
BTW, the sound acutally helps me set my end points.