MacBook boots up to grey screen and nothing else?

I picked up a MacBook mid-2007 2.1 GHz from a friend. He said it wouldn't boot up and Mac Authority wanted $500 for a bottom case replacement. The MacBook is in decent condition but when the power button is pressed it boots up to a white screen and nothing more. I have tried the standard troubleshooting techniques, safe mode, external hard drive, install disc, PRAM, etc. and none of it does anything. A chime can be heard when booted up and i have tested all the connectors but it does the same thing each time. The only thing if find weird is that there is absolutely no hard drive activity. You hear the platters spinning but the arm doesn't move. I have tried different hard drives and they all seem the exhibit the same behavior. My next step will be to swap logic boards with an identical MacBook and see what that does. If anyone has had a similar issue or can shed any light on the issue it would be appreciated.

Thanks