An air pump will usually only run for a max of 100 seconds and it's pitch is constant and does not increase with engine RPM. A seizing tensioner or idler pulley on the other hand will howl big time, only when cold, and will go away after a few minutes. The one that I changed out last week was the timing belt idler pulley and it was so bad that it would barely turn in one direction and locked up solid in the other. It was only a matter of time before it seized up entirely and burned the belt off.

Get it stone cold and take the timing belt cover off and then fire it up. A half ways decent set of mechanic's stethoscopes are cheap at the local Harbor Freight. With it running carefully stick your probe into the center of each bearing where it is bolted down and not spinning. It should be real obvious which one is the culprit.

Mark