It may be feedback in the lower frequencies.

Experiment with the tone controls on the monitor (if there is any). Turn down the bassier frequencies a bit.

Or, move the bass cabinet itself further back, or to the side. Just a foot or so. See if this helps. This is assuming the feedback is coming from interaction between the pickup and the bass cabinet.

It could also be coming from the PA system! For instance, if the bass cabinet is pointed at a mic, certain frequencies may cause occasional feedback.

Rich McCarthy

This is assumin