Sounds like strings hitting pole pieces to me.
I cannot unequivocally state that it has no side effects (though I've not experienced any), but it's become standard practice for me to put some clear nail polish on any exposed pole pieces on my basses. As a side benefit it stops them from rusting, too. Obviously if you're getting a lot of contact it's probably not going to be enough, but I've found it helps dramatically, and will stop that unpleasant pops that can happen when you really dig in.
I am Jack's neglected P-Bass|This is your gig, and it's ending one minute at a time|Yeah. Well. Whatever. You can't teach a guitard anything.