I had my oil changed at Jiffy Lube and they put a bit too much.
How much extra? And I take it that it didn't knock before you had the oil changed?
Do you have a CEL?
You could have incorrect valve clearance, worn valve rocker, worn cam, or broken valve spring. However, it sounds like something a little deeper in the block. You said you have a stethoscope, so, locate which cylinder(s) is/are noisy.
Disconnect the fuel injector connector of the noisy cylinder and see if the sound dissipates. If it does, it’s possible you have a worn crank main bearing or worn bearing at crankshaft end of connecting rod. If it doesn’t, it could possibly be a worn valve lifter, cam gear, or worn cam journal bore in the crankcase.
Have you ruled out the knock sensor, bad gas?