What causes the ringing?

[urchin]

Is the noise from harmonics, like in this video?

YouTube - muting strings - bass lesson

(if you cut to around 1:30 that's when you can start to hear it)

 

[Jay2U]

These muting techniques look very familiar. The difference is that my bass is much more alive. Not to mention what it does with roundwounds, but I'm a ftatwound player anyway. It appears to me as if the J-bass from the video has notably less sustain.
Thanks and keep playing subsonically!

 

[mongo2]

Thanks "ishouldbeking". It's no rattling. It's always present, also unplugged. The E-string is most affected. Playing a flagiolet above fret 7 on the A-string causes the E-string to ring as loud as the A-string. Playing a flagiolet on the 7 th fret on the D-string doesn't have this dramatic effect on the A-string. Between G and D there is some effect, but not dramatic.

It's probably due to the resonance in the bass exciting the harmonics of the resting string. That would also explain it's selectivity.