MacBook Air won't turn on - it only hoots !

Question

My MacBook Air (2009/2010) will not turn on: when I press the power button there's a tinny little hooting sound  every 5 seconds, exactly in sync with the flashing power light - that's all - no sound of fans or anything, nothing to see on the black screen.  Any ideas ?  Thank you !
FC234B/A RFB MACBOOK AIR 13.3 /2.13GHZ/2GB/12-GBR
bought refurbished from Apple June 2010
I have tried the following Apple Support advice :
If your MacBook Air doesn’t turn on or start up
Try the following suggestions in order until your computer turns on:
Make sure the power adapter is plugged into the computer and into a functioning
power outlet. Be sure to use the 45W MagSafe Power Adapter that came with your
MacBook Air. If the power adapter stops charging and you don’t see the indicator
light on the power adapter turn on when you plug in the power cord, try unplugging
and replugging the power cord to reseat it.
Check whether the battery needs to be recharged. If the light on the power adapter
glows amber, the battery is charging. See “Running Your MacBook Air on Battery
Power” on page 34.
If the problem persists, return the computer to its factory settings by pressing the left
Shift key, left Option (alt) key, left Control key, and the power (®) button
simultaneously for five seconds.
Press and release the power (®) button and immediately hold down the Command
(x), Option, P, and R keys simultaneously until you hear the startup sound a second
time. This resets the parameter RAM (PRAM).
Resetting the System Management Controller (SMC)
on Portable computers that have a battery you should not remove on your own include MacBook Pro (Early 2009) and later, all models of MacBook Air, and MacBook (Late 2009).
Shut down the computer.
Plug in the MagSafe power adapter to a power source, connecting it to the Mac if its not already connected.
On the built-in keyboard, press the (left side) Shift-Control-Option keys and the power button at the same time.
Release all the keys and the power button at the same time.
Press the power button to turn on the computer. 
Note: The LED on the MagSafe power adapter may change states or temporarily turn off when you reset the SMC.

Answer 1

thank you - it's just one tone, which lasts half a second (every five seconds) - and with exactly the same timing, a single pulsing flash of the white power light comes on and goes off..