There are several things that could be happening here. It could be a loose crank arm, but it could also be the chain slipping off the gears.
Is this a bike with derailers (shifters front and/or rear)? If so, is the chain on the biggest front gear and the biggest back gear, or vice versa?
Is the chain lubed? Is it rusty? You might want to lube the chain. Don't use WD-40.
Also, you might have a bent or broken tooth on one of the gears. Check them out.
Good luck finding the solution fast.
Jim