I've started playing bass using an old one my father owned. It has not been serviced for ages as far as I know. I've plugged it in to a new Vox Pathfinder practice amp with a new cable.
I notice that as well as some crackles from the knobs (expected) I can get a pretty loud hum when I hold it with all strings open. If I mute them all, it stops. If I remove my hand, it builds up over a few seconds.
Is this feedback resonance or mains-hum or something else that needs fixing on the guitar? I am not playing with a particularly loud amp and am probably about 6 feet (2m) away.
Update: I was interested to see what makes the hum start and stop. I discovered that touching any individual string does not make much difference. However touching the bridge does. Someone mentioned grounding Vs resonance, I'm not sure what this tells you because equally, touching the bridge might be enough to damp the resonance?