We had a similar issue with our I20 while racing the Tybee 500. It was pretty freaky. However, we had determined that the noise was caused by shifting of the shrouds at the upper tang on the mast. The sound was transmitted to the hull through the wire making the hull resound like a drum. It sounded like that's where the problem was but I investigated several times with my hand on the shroud and it was not hull related.
I recommend re-rigging the mast hound and see if the sound goes away.
Given what you have said, I would go with the above....
Take the mast down, check everything, make sure nothing is crossed up and put it all back up again. Will then probably have gone away.....
