Symptom of Timing Belt or Sprocket Problem?

My 2000 Prizm (102,734 mi.s) has started to emit this persistant very audible mechanical whirring sound, and it seems to be coming from the engine block itself (not from the serp belt or its pulleys, for instance).

Is this just a typical sound for a car with this many miles, or might this point to a potential problem with the timing belt and its sprockets (or something else)?

Thanks!

John

Whoops--I meant timing chain. Sorry.

John

I gave up and took it to the shop. The A/C compressor was bad--a part inside of it wasn't turning normally but instead was wobbling around inside the housing, boring a hole that allowed all the refrigerant to leak out (and making that metallic whirring sound. The whole shebang had to be replaced.