Detune the guitar a whole tone, if the problem is now with your B string then it’s the guitar body resonance, maybe a crack in the body or loose back or the resonant frequency.
If it’s on the A string only then check for a loose tuner, check the contact points of the A string (nut and bridge), clean them up and make sure the string fits in the grooves well. You could also try a lighter gauge A string, it will be slightly thinner.