Rapping on the Body
So, how about a little "rap?"
Mechanical buzzes occur as a matter of direct or sympathetic vibration of various body or neck parts of the instrument.
When the string is ringing, the top and body vibrating, it's often difficult to isolate the source of the buzzing.
I find it is frequently possible to pin down the location of loose parts by rapping all around the body, concentrating on the likely areas of looseness around the perimeter and over braces:
I hit it pretty hard with my knuckles to hear the "flap" of a loose top, back or brace. I can also hear the vibration of attached parts this way.
The same goes for the neck:
Rapping on the back of the neck will often reveal the rattle of a loose truss rod. I have to hold onto, or remove all the tuners to avoid being confused by their rattling when I'm testing for a loose truss rod. Speaking of the tuners, I can rap on the peghead to reveal looseness there, too.