Thank you all so very much for your thoughts!! They are all greatly appreciated (and humorous, in some cases).
We will keep up the investigation... we're pretty good about lubing and checking things each ride, but it's always possible that we've missed something along the way. We hadn't thought of the wheels (though we did have a broken spoke not too long ago, so maybe those need to be checked again). I would think if it was a wheel issue, even if we weren't pedaling it would still make the noise, but I think we just need to eliminate everything as a possibility, so certainly worth checking those as well.
Again, thanks to all for your thoughts. Always greatly appreciated!!