Schwinn Johnny Pro Stationary Loud Noise Near Sprocket
From your description it sounds like the 'bottom bracket' bearings have become damaged. These are the bearings that the crank axle runs in in the frame of the bicycle. 

To confirm, turn the cranks by hand, if you feel an area of extra resistance, any 'grinding' or a 'notch' or 'clunk' that the cranks pass through the bearings need replacing.

I found a lo-res exploded drawing here of the Johnny Pro, it indicates that the design uses a 'three-piece' crank (two separate crank arms, axle and bearings in a cartridge type unit). The cranks typically fit onto a square taper or splined interface on the axle. If the bolts at the  base of the cranks are removed the  cranks do not come of the axle, a special crank puller-tool is needed for that. The cartridge axle and bearings thread into the frame but a special tool is needed to interface to them to unscrew them.

Here's a video that shows how these things come apart.

The bearing cartridges themselves are not very expensive. You'll have to make a determination if you want to try this on your own or get a bike repair store to help you.