Qhy do i hear knocking inside the engine block.?

CK OIL LEVEL IF GOOD HAVE IT CK'D BY YOUR MECH.ANY CODES?

Blown head gasket or cracked head, letting coolant into piston cylinder will make a lot of noise like a blown engine. Did you lose coolant level in radiator but none going on the ground. coolant will go thru the exhaust system, this will damage or destroy sensor and cause engine light to come on. If it a small leak at gasket try radiator stop leak "aluminum dust or walnut stop leak from ac delco" be carefull to not use Glass stop leak or you will stop up all coolant ports in your engine. Impala has a Recall on head gasket on older models. You did not say if you had any codes.