Is the lead-acid battery connected? If so, pull it out. If it's developed a shorted cell, it can load down the power supply and make it go into periodic current limit. This will prevent your HD from spinning up, cause clicking sounds, and make your backlight flicker a couple of times per second.

This can also be caused by a defective power brick, so if you have another one handy, try substituting it for the one you're using now.