I'll try that but I've never opened up a laptop before so it's a bit daunting.You should take off the cover if it's easy enough to do it on your own and check and see if one of the fan blades are broken? Broken fan blades can cause that I'm not to sure myself.
Or maybe the fan itself is bad, who knows. Try holding the fan and not letting it spin and see if the chirping stops.
I don't think that's the case since the frequency is once every 2-3 seconds although it's somewhat irregular. I can also hear the normal HD activity sounds when it happens.Is it not the HD head seeking/stopping? My WD scorpio makes that chirping noise whenever windows spins the drive down.
I've heard of similar noises coming from laptops. Almost a high-piched whine. Turned out disabling the CPU power saving features in the BIOS solved it.
Good luck!