Well, after a week, they fixed it. The official report states:

"A THUMPING TYPE SOUND IS HEARD WHILE VEHICLE IS AT IDLE.
THE IDLE DROPS DOWN CLOSE TO STALLING AND RISES BACK UP.
IDLE IS NOT CONSTANT. NO WARNING INDICATORS ARE ON.
FAULTY TCS VALVE AND THROTLE BODY ASSY.
PER TECH LINE REF#1304494, REPLACED TCS VALVE WHICH DID
NOT COMPLETELY CORRECT PROBLEM. THROTTLE BODY WAS REPLACED
NEXT WHICH COMPLETELY CORRECTED PROBLEM. TEST DROVE VEHICLE"

I am not hearing the thumping sound anymore, and the idle is steady now at about 800rpm.

Any know how to get the info referenced by "TECH LINE REF#1304494"? I'm curious to see exactly what that says.