Thanks for the advice, Allan. I'll borrow one and give a listen. I had tried listening with a piece of aquarium tubing, but couldn't get a good connection. This vibration seems metallic, though. When I had the strings off I did a pretty thorough thumping of all wood surfaces and didn't hear anything suspect. Maybe the stethoscope will reveal more.
Just noticed I forgot to put HTML codes in my original message. Pretty hard to look at...