Dishwasher Sound

My Frigidaire dishwasher (m/n: MDB532BR0), built in 1995, is making a loud sound each time I start it. The sound continues for 20 - 30 seconds, slows down in speed and then disappears. In every other respect the machine works perfectly: it fills up, washes great, empties, and dries correctly.
The sound is coming from the bottom of the machine. I removed the lower panel and can see a \\\"ring\\\" around the rotating shaft, just above the motor. It \\\"vibrates\\\" in sync with the sound on startup, and stops when the sound ends.
Questions:
1. Anyone know what it might be? Some type of \\\"clutch\\\" component??
2. Anyone know where I could get a manual with exploded-diagram and/or schematic?
I Googled Frigidaire and the model number (doubled-checked for accuracy) and got zip. In every other respect it works fine. I\\\'d like to fix it before it affects machine performance.
Thanks in Advance!

Hard to tell from here exactly where the noise is coming from. Perhaps it is a loose or broken impeller? If you go to partselect.com and then plug in model number MDB532 that should give you a breakdown of your dishwasher. It worked for me anyway.