Pretty sure the sound is resonating, if there is a bad bearing in the serpentine belt assembly or a fan/ac compressor it should be louder with speed. If you are not facing any check engine light or malfunction (poor performance, fuel economy etc) then it's just a mechanical glitch nothing to worry about, Unless you think it might leave you stranded somewhere or may cause major damage in future i see no point in having it looked or to spend money diagnosing it only to find nothing. At best it would be from exhaust (tighten those bolts) or some rubber is cracked somewhere and the metal is vibrating at a certain speed range.
