If it's a metal grill, remove the grill and install rubber washers between the grill and the cab. Is it on casters? Could be a rattle there. Or it could be something loose in the cab. Open it up and check that all the speaker screws are tight. Make sure the jack plate is tight.
Precision Bass Club #1070, Jazz Bass Club #1145, Rickenbacker Club #499, G&L Club #512
Gibson Club #268, Carvin Club #290
Short Scale Club #400, Fretless Club #860