Clunking noise from back of the engine
 

I just changed my oil from Mobil 1 synthetic to Castrol synthetic today and the engine started making a clunking/knocking noise. I know its not from my cats because I dont have any.. The noise is comming from the passenger side of the engine near the firewall, sounds like tapping from inside the engine. Oil level has been checked and is good. Any help would be appreciated.