Non-contact voltage tester beeping near anything metal in my house?
It's probable that the 'non-contact voltage tester' is responding to a strong RF field, with the metal objects acting as 'retro reflectors'.
The sizes of those objects point to a VHF / UHF transmitter nearby.
It would most certainly not be a grounding issue since the metal beading of a bench, guitar strings, a metal rod and a metal hinge would not be even remotely connected to anything electrical.
The presence of a strong RF field, and its frequency, may be confirmed using a digital field strength meter.