PS3 Slim Makes Random Beeping Sounds

My 2 yr old PS3 Slim 120GB makes this beeping type noise randomly. It is a short, high pitched beep. It is not in a closed cabinet, and the front indicator light has never turned yellow, nor does it flash yellow when I'm directly in front of the console (when I catch it beeping).

I've never worked the console hard-no heavy game playing, well, in fact, none at all in months. It happens mostly when streaming, but a couple of times, it beeped when eject a disc.  There is never a disc inserted when it beeps. Hard drive is less than half full. Hard wired with ethernet cable.

Any ideas why or what this means?

It's a single short beep.  It's not a squeal...Again, there's no blinking light-which I'd hope would blink if it were overheating and the console's not sounding any louder than normal.  I just wish I could find out what's causing it.  It seems to happen when streaming Netflix-I streamed some video from the PC for a couple of hours last night and nothing. Then 30 mins into a Netflix movie it beeped.  It has happen when not using NF, but that's when I notice it happening the most.

Thanks for your help though.