engine making a loud knocking noise

Clean the idle air control and EGR valve and see what happens -also check the MAF and TPS sensors

You can do all that. But it's also likely to be caused by a break in the hose from your Brake Booster to your upper manifold. If you notice that it's a pain in the foot to apply your brakes, then the two problems are linked. Get a new hose and you'll be good to go, or it might be only cracked at the end where it connected to the upper manifold.

Also, it may come down to a simple rubber elbow that is located on the back right (as you face the car) under the air intake manifold, over the dipstick for the trans fluid. That coupled with a busted hose to your PCV valve is responsible for your vacuum leak. Check all the hoses in that area. Some will likely be cracked, leaking hot oil gases, and/or come loose. There are 2 elbows underneath and one can come loose very easily as the oil breaks down the rubber material.