to test just the crank, get on the bike next to a street sign pole you can hold onto... while stopped, stand tall on the pedals. put one pedal forward so the crank arms are horizontal, and bounce your weight on them, then backpedal so the OTHER arm is forward, and bounce your weight a bit again, repeat a few times. does it creak when you do this? if so, its highly likely to be the bottom bracket or the crank attachments, especially if this bike has square taper cranks.