Are you talking about high-pitched feedback? Sounds like it if you have sensitivity to the treble control.
It's unlikely that your pickups are microphonic (Alembic stuff is potted to prevent this), but that's often the cause of a whistle like this. Easy way to check is to see if it does it with both pickups - much more likely that one pickup is defective than both of them.
If the problem is on both of the pickups, then it may be electrical feedback, which could be a problem in amp or bass. If you're having amp problems, of course, it's more likely to be there. Typically some component is out of spec and causes there to be way too much gain outside the normal bass range. I guess it's actually more likely that this sort of problem might show up after your amp is fixed if the tech didn't have the exact part or was unable to do the full calibration.
It's possible that it's just regular old feedback, but it would have to be so loud for this to happen (especially at high pitch with bass speakers) that it just seems there must be something wrong.
David Fung
