So, when you touch your guitar strings, it’s you that’s being grounded through the guitar. The reason the noise quietens is that when you’re grounded, you no longer act as a fleshy meat antenna (careful, now). Ordinarily, you’re standing around soaking up various electrical interference from the environment.
Furthermore, how do I fix a buzzing bass string?
Then, how do I know if my guitar is grounded? Checking Your Guitar’s Grounding
This is a simple thing to test for with your multi-meter. First switch your multi-meter to the Continuity setting. Touch the probes to a guitar string and the ground section of the output jack. If you hear a clear beep then your ground is good.
Also to know is, how do you get rid of hum on a bass guitar?
Why does my bass guitar make a buzzing noise?
Fret buzz is generally caused by some combination of not enough bow in the neck, the bridge string height being set too low, and possibly a warped or irregular neck. Other important concerns are consistent fret height over the length of the neck and the type and thickness of strings used.