@faizan920 you are probably mentioning the crankshaft sound... sound from the bottom of the engine... knocking is generally referred to the sound of early ignition timing specially heard on petrol at acceleration.
Please clarify, to get relevant answers.
If its the crankshaft sound, its probably the worn out bearings. Usually happens in overheated engines or oil lubrication problems.
Knocking can be due to bad ignition timing, improper fuel injection, etc