After I turn the car off I there is a fast paced ping or knocking noise. I was wondering what causes, how do I fix it and is it Serious. Thanks

What engine? Possibly idle is too high.

Its a 4 cyl. 2.2L auto. Also I recently replaced the plugs and right after the noise seemed worse and more fast paced.

What it is called is "preignition". Get the exact plug number from the dealer and replace the plugs with those just as a test.
You can check your timing but it may not be adjustable. It is set exactly by the crank TDC sensor.
But it does sound like the wrong heat range of plugs. Parts counters are notorious for not looking up the right plugs for the engine. So for this one time check with the dealere and do NOP cross referencing.