This is a new problem for me. I bought a fretless T-40 about two months ago, cleaned it up, new strings, setup to new gauges and so on. There's a buzzing (sitar type noise) at the 4th, 5th, and 6th frets unless I play very gently-- other areas of the fretboard do not have this problem. The bass had this problem before I did anything to it; I thought new strings and a setup would fix the problem, but it hasn't. The nut looks ok and I ensured the strings are not twisted. The neck is not visibly warped. There is some wear to the fretboard in this area but not enough that it looks like the cause. Any suggestions?