"Clacking" noise coming from drivetrain
I had a similar problem until recently, initially thought it may be the BB, but after cleaning, greasing, and eventually replacing the problem was still there.

Turns out it was the freewheel hub on the wheel, and fairly quick to fix, if that is your problem, open up the freewheel hub, I needed 2 5mm allen keys for this, but various wheels have different methods of opening up.  Don't force anything, these are easy to destroy, and expensive to replace.

Once you have the freewheel open, make sure that the little pins stay in place, most new hubs will have springs keeping them in place.  

Apply grease, I used the Park Tool one (personal preference), and close everything up again.