Bass is a noisy instrument.
If you do it too slow, it'll make a noise. If you do it too fast, it'll keep ringing. You need to find the time where it does the least noise but still stays mutes.
Also, check if it happens on all frets or just an area (Might need a set-up).
And last thing: Raise the volume. Often noises aren't picked up by the pickups.
