I have a hissing noise in my 04 Durango under the gas peddle. It is prominent when I let off the gas over 1000 RPM. When it falls below 1000 RPM it goes away. It sounds like a vacuum leak. I have driven other Durango's and they don't make the noise. Dealer says it's normal. It may be normal to have an exaust system of some sort, but not to hear it. I can hear it over the radio. Any ideas?