The most likely cause is that something is resonating when ever the C note is played. If it sounds like it's coming from inside you can use a chopstick to (gently) touch internal parts to isolate the noise. It may be easier to tune the A or D string to C so you don't have to fret a C note while doing this, this step will also rule out fret buzz as the source. A small make up mirror taped to a chopstick can help you see inside the body.
Possible sources are: A loose brace, a loose wire (if you have a pickup & preamp system), things like that.
Well, for locating resonating parts in instruments, a cheap vibrator with soft/rubbery surface can be quite useful. The surface consistency is for feeding the vibrations into the instrument's body rather than having the thing rattle/snare at contact point. In a pinch, you can likely use a rubber chicken as intermediary. Just make sure that nobody else gets to see the contents of your instrument repair chest.
Actual bona fide back massagers tend to be too much on the strong side to play well with instruments.
The G string started buzzing annoyingly mostly when fingerpicking on my Takamine electro acoustic. Then it spread to the D string which was even more annoying. Removed both strings and checked all the fixings before reattaching only to find the problem was still there. Strangely enough the buzzing only occurred when the guitar was in the playing position. Placed on its back, there was no obvious buzzing from either string. Started me thinking it was a loose interior fitting so I started with the preamp which appeared pretty secure. However when I removed the battery and played the two strings, I noticed that the buzzing had mysteriously gone away. Tried out the preamp with another battery and still no buzzing strings, so I can only conclude that the issue was with the original battery which was not seated properly in the battery compartment. A lesson learned, though…