Electric guitar: 5th fret, 2nd and 3rd string "plunks"?

Take it in to get it strobed and the intonation redone. A little in between string maintenance goes a long way as well. Buy some .0000 steel wool and LIGHTLY scrub your fretboard AGAINST the grain to clear away any grime and debris which will give you that 'plunk' sound. Finish that off with your favorite brand of lemon oil to quench your fretboard's thirst. I hope that helps.