Knocking in your engine can mean something severely wrong or even knocking can mean something simple as the wrong fuel or a fuel problem not burning properly in the engine.
Sometimes engines can make knocking noises due to the air and fuel mixture not being correct which causes the vehicle to knock because the fuel and air mixture not igniting properly.
In that case the problem is usually a simple fix.
However sometimes you might have a more severe problem especially if the engine knocking doesn't go away while driving.
Engine knocking can also be caused by bad rod bearings that are not getting enough lubrication or the rod bearings could be wearing out.
If your rod bearings continue to go bad and you drive with bad rod bearings with that engine knock then you could either throw a rod or have a spun rod bearing happen.
When you have a spun rod bearing or throw a rod then your engine can suffer damage and your crankshaft will be damaged as well after a spun rod bearing.
If your engine is continuing to knock you should get the engine checked out to be sure it's not a more severe problem such as the rod or main bearings.
But sometimes like I said it can be as simple as a fuel to air mixture not being correct or the fuel you're using in your car may not be compatible with your engine.
Using some certain fuels with the wrong octane level can cause engine knocks.