MBPr 15 Late 2014 - 1 Beep ? driving me MAD

Hi this is driving me mad,

Specs : MBPr 15" 256GB/16GB Quad 2.2 i7 model MacBook Pro (Retina, 15-inch, Mid 2014)

Whilst using Unity3d I hear a random 1 beep ( chirp like the 3 beep's )

At first i thought the Mac was overheating but the Fan controls app i have says cpus are fine and cool down ok.

However I had a hunch it may be the GPU temp ? this idea may have been thrown out as from a cold boot with 2 USB HDD's plugged in I got the single beep one more time after account login ( when the little white bar goes across the screen 3/4 of the way then stops then 1 beep and then continues )

Apple hardware test Power on + D all passes.

Does anyone know what this 1 beep is ?