Hearing this too, then saw this thread. Very odd. Seems to be coming from the bottom underneath the trackpad. It is NOT the HDD parking heads. I have an SSD in mine and I hear it.
I think it might be the bottom cover, and tightening or loosening the screws under/near the trackpad might solve it. Will try it when i get time.
for me, it will happen at a random time when i move the machine, but then when i try and reproduce it to record it or something, nothing happens. just happened 2 times when i lifted the right side of the machine, but cant get it to go a 3rd time. for me, seems to be coming form the superdrive. too loud to be heads parking.
im hearing this too, im taking it to the store today