Considering Pluck & Bow said it sounded like it was coming from inside the fiddle, I'd say it's likely something's come unglued. I played someone's fiddle last week, and got a buzz. The top was separated from the sides along about an inch-long stretch. It DID sound like it was coming from the inside, but was quite visible. I'd think if a sound post was loose enough to buzz, it would simply fall out. Maybe not, though, and they can dry and shrink or wander to a less-than-optimal location. Buzzy strings/bridge/tuners sound (to me anyway) like they're coming from the strings/bridge/tuners and not inside.

Sometimes, if there's a loose brace inside, you can tap the top at the edge and hear/feel whatever it is inside tapping back.

Oh - one other mysterious but easily remedied cause of buzzes: hard bits of stuff on YOU that vibrate on the fiddle. Necklaces, earrings, buttons on clothing. These things vibrate on the back of the fiddle, so they travel through it and can sound like they're coming from inside.