I can't find it period. It comes and goes at will and regardless of which string I pluck or how I hold the instrument it is present WHEN it want's to be. It sounds like a string rattling against SOMETHING, but I can't find it if it is. Sounds like a loose endpin screw but it isn't, at least not when I check the screw. I am a novice with the upright in the respect that I have only owned this bass for a month. I am very familiar and intimate with it though as I have spent hours and hours setting it up. Please anyones suggestion on where to check would be so, so ,so appreciative. This rattle only happens on occasion (Sometimes 3-4 times in an hour though) but when it does it drives me nuts. Thank you. P.S. The bass is an Engelhardt Swingmaster-It has TI Spirocores if that helps.