This happens often but I can't cause it on purpose so I haven't brought it to mechanic since I am not sure I can cause it and it is quite quiet. I've 95K miles on manual 1992 Sentra. I am in 3rd gear, slowly accelerating from 35-40 mph if it occurs. There will be 1 short, quiet whistle, almost like a bird call or the last slurp from a straw. My husband thought it was the tires. So I got the whole package: 3 new tires, rotate, balance, alignment. Problem persists.