Could be tuning keys, could be trussrod, could be the bridge saddles not 100% stable (see if they move around), could even be the nut.
If it's the trussrod, the sound will come out of the neck pocket (usually).
Put your hands on various bits of the bass, and see if it still vibrates.
If it's the trussrod, the sound will come out of the neck pocket (usually).
Put your hands on various bits of the bass, and see if it still vibrates.