If it were rod knocking, it occurs even at idle. That is rather low for idle on any car. If your car is in the lean fuel setting, sounds like a Heated O2 sensor or catalyic converter is about toast.
Since your car is in "open loop" status, your PCM is on running on KAM (Keep Alive Memory) (learning this now at school *UTi*). That is whats causing your car to idle a bit low, but its doing its operation once detected a voltage malfunction through a sensor. That tapping could be either a cam knock sensor or crankshaft knock sensor. I cant pinpoint without actually mechanically diagnosing the car, but for Generic OBD II scanners, it will point you towards what system(s) are affected.
I hope this helps and you should have it checked out by a GM certified technician, not a mechanic.