Savarez 5212R 12th string (F) standard tension, Single String, .082 in

This is a brilliant idea to make a bass string fit on a classical guitar without drilling any new holes. It has appropriate tension and even seemed to have the right action without filing the nut or bridge. However, the bracing inside my guitar, which normally seems solid, vibrates and squeaks in an unpleasant way when I pluck this string. I think the bracing is loose or not glued down properly but the problem doesn’t become evident unless playing these very low bass notes with heavy strings. (My guitar is a cordoba C5. ) I thought I could find where the brave is loose and glue it down but I was unable to figure out where the problem lies. So I took this string off and put it on my other guitar, which is also a cordoba c5, but 20 years newer. Unfortunately that one had the same problem: bracing makes vibration sounds when playing low bass strings but not with other strings. It’s not a rattle or a buzz sound. It sounds like “SSSS”, like a hissing sound of wood moving against wood. I still have the string 5 stars because the design and construction of the string seems flawless. It’s the cordoba C5 that should loose points. I still love cordoba guitars though. I can buy the c5 used for under $200 and it sounds and plays better than other guitars that cost four times that price. Just don’t try to use it for a bass guitar.