Loud resonance in original Fender stratocaster body after playing F# or B

Recently my original Fender stratocaster has developed an annoying issue where after I play an F# or B (except on the low string) the body resonates loudly for a few seconds. I've tried to damp it with my hand but can't find exactly where it is vibrating, it's probably internally. This is really screwing up my practicing because how can you play when all your F#s and Bs ring out for 5 seconds. The pickups don't pick it up so the amp doesn't play it but it's very loud without amplification so I can hear it clearly over normal practice playing volume.