As Kimmo said, the best approach is to narrow down the list of suspects by isolating them one or two at a time. For starters remove the chain from the chainring, and hold it up out of the way as you spin the crank. If it's still noisy it's either the bottom bracket bearings, or maybe the pedal you're holding, otherwise it's somewhere else. Continue working this way to check other places, ie spin the wheel with the chain not connected, etc.

When you've narrowed down the range of possibilities focus in on what's not eliminated and you should be able to identify the part,if not the exact cause and solution.