Strange noise coming from bottom bracket/crank
If you only just bought it from a shop, take it back and get them to sort it, but do ask what the problem was.

If you want to fix it yourself, it's worth determining the frequency and type of noise that you're hearing. I used this site when I had an issue similar to yours and I tried taking pedals off, the crank arm off and eventually took it to a bike shop and it was the bottom bracket that was making the noise.