if it's a brace (x or finger) stew mac makes little jacks that can support a glued brace from the inside. or you might be able to fit in a bridge type clamp. or you can use a large bolt and a nut to use as a jack screw but be careful with how much pressure you apply. make sure the gap is as clean as possible before gluing. squirt in some de-ionized water into the gap with a syringe then carefully blow dry with some canned (or other) air. repeat this a couple of times but go easy on the water. you can use a thin metal spatula such as a fine art painter would use on a canvas or again the trusty syringe to get some glue into the gap. my glue preference would be titebond original (yellow) or LMI white.
this would apply if you have a brace that's come loose from the top. if the brace is pocketed into the kerfing and it's loose here, then you'll have to get creative with the clamping.
i know it sounds silly, but could there be a air pocket under the pickguard that's causing the pickguard to vibrate about?
hope it works out
