Display flashes once but MacBook doesn't boot
Press power but no response other than display flashes once for a second and then goes black.
No chime or front sleep light or hard drive spinning up.
If I flex the bottom case upwards a bit, it's very similar, i.e. display flashes once, but this time the Caps and F6 key light up. And there appears to be some sort of faint hum from inside the Macbook. But no HD or DVD spinning up.
I took this in trade for spare parts from a guy who described the MB as not starting up, except that occasionally he could get it to boot by flexing the bottom case. And it would stay on for days at that point.
Any ideas? Thanks.