RIB SEPARATION!

The arrow points to an area where a sheet of paper can slide in between the rib and the soundboard.  This is just enough room for the the rib and soundboard to buzz against each other...but in this case, only when the D flat above middle C is played.  Once the humidity changes a little bit, it might not buzz at all.  Maybe tomorrow it will show up on a different note.  This is common.  Fun for the whole family!