Noise My 1999 Dodge Ram V6 Automatic Pickup Is Making

For the last 6 months I have been hearing a squealing type of noise from under my truck when my gas pedal is pressed down in between 30 mph and 45 mph mark. If you take your foot off the gas pedal letting it go down or going up past the sound then it disappears. It doesn't happen when I'm parked in my driveway reving the engine trying to bring up the noise again to try and isolate the sound. There is no rattling or vibration of the truck either when you hear the sound. It drives normal just you hear the squealing or whining sound it makes till you go past the sound then it shuts up. It just happens betwene 30 mph and 45 mph and you either got to let your foot off the gas to make it disappear or press the gas pedal down more to make it disappear. If I leave the gas pedal held down constant where the sound is at it just keeps going and going till you get off it. Brought my truck into Goodyear when I had my shocks replaced and my manifold gaskets replaced and told them of what I was hearing. One of their technicians drove the truck said it had something to do with the cowling under the truck around the muffler like a firewall metal plate they described it. All I know it is annoying and wondering if it is something to really worry about or not?