Sounds pretty darned normal to me too. Don't really know why you were concerned.

 

I have a 2013 Slim and that's exactly how mine sounds. I leave mine plugged in most of the time and that's still the result. I've been running the same battery since January of 2014 so I'll probably replace it here soon just to be safe. Batteries seem to be the culprit more often than not if you're having issues.

 

compared to mine,, thinking you're good to go...