Hi and welcome to the forum!
Take a pick and gently tap on the bridge, middle and neck pickups while the amp is at playing volume. If you hear a "click" tone with the bridge pickup which is noticeably louder and more metallic than with the other pickups you have a microphonic pickup. Take the pickguard off (or let your tech take it off) and check whether the bridge pickup has a metal base plate. If so, check whether it is solidly attached to the pickup - if it is loose, that can cause squealing problems. The solution then is either to remove the base plate or to solidify the joint between pickup and base plate. Since I have never done this I cannot give you a detailed description of how to do it.
If you don't have a base plate you may have a microphonic pickup. This can be cured by wax potting the pickup. I believe BKP does this service, too.
Cheers Stephan