Aww, all's well in the universe once again, then.
This thread reminds me of my own experience with string buzzing - on the upright 4. Fingering a low C (on the A string) always produced a noise. So I dismantled the bridge, checked it thoroughly, cleaned it, added graphite into the grooves, cleaned the strings, made sure the tuning pegs were fine, then riassembled the lot. Now ALL the strings buzzed like mad!
The cause - I finally discovered - was a loose tailpiece.