93 accord ex bad noise at certain rpm's

i have a 93 accord with 240k miles on it...recently i heard the car make a high pitched whistling noise u could barely notice at first.only at 1,900 rpm..then later on it kept getting worse and worse now its anywhere between 1,500-2,450 rpm..at first i thought it was the exhaust because it kept making the noise of a rusty muffler,and its hard to determine where exactly the sound is coming from. but i lifted the car and the exhaust was fine...than i also here the noise even when i hit a speed bump or a small hole in the ground...so i figure now that its something loose but what???

if anyone had the same problem or any suggestions PLEASE HELP..i dont want to get stranded.its my only car and she's been good to me