You can do a cylinder balance test if you have a scan tool.
If not, you can disable the ignition or fuel injector on one cylinder at a time checking for a change in noise. It may be easier just to disconnect a fuel injector harness connector on one cylinder and see if you hear a notable change, if not, reconnect and go to the next cylinder. If the fuel injectors are hard to get to, you could disconnect an ignition wire, ground it, then again check for a change in noise while at idle. If you are unsure on how to do that, just Google it. And don’t get zapped….
Do you have a mechanics stethoscope that you could use to help locate this particular noise? Also, make sure your ruling out any other noises coming from the engine bay from accessories and so forth.
I really doubt its a rod knock or anything that major. Its possibly just excessive lifter noise.