Engine knotting

If it is a hard knock, then that means that the rod bearing went out and has too much play just like moparownr said, but if there is a noise like a little "tick", that is just a lifter tick and is no big deal, its common on chevys

If you engien is knocking, it means usually that a rod bearing has given out and too much play exsists between it and the crankshaft. This can be repaired, by rebuilding the engine, depending on mileage and damage the crankshaft will probably have to be replaced. So even though it can be repaired it may be cheaper to install a new (rebuilt) engine, also should be a faster repair. Be choosey and select a quality shop.