The squealing you hear is the fuel pump. The service manual has testing procedures for the STVA. C28 tells you what system has the problem, not the specific component in the system. Based on the way it's moving, I think the secondary throttle position sensor is suspect. Sometimes they short internally and fool the ECM into thinking the STVA is moving erratically when in fact it's just bad data. The ECM just doesn't know the difference.
However, that STVA is a one off and was only ever used on the 2003 600. It's a bit of a goofy design compared to all of the other STVA designs.