You may not solve noise problem as it will likely move on to somewhere else. All car's body will flex to a certain degree. Designer are trying to make car's body more rigid and use suspension as the only moving part. Thinking of Go-Cart that use body as its suspension. My point is that your car will feel different when you put a 4 points brace on the monocoque floorplan. Not sure if you'll like it or not. If you don't plan on racing your car, I wouldn't put it on. A stiff car maybe less fun to drive. Talking from my experience with putting braces on many of my previous Subarus. Roll bar, upper/lower strut tower braces will do the same. However, if your car has been abused and body has too much flex in it, then putting brace on will help.
