I have noticed ever since I owned my 97 C1500 that after a certain speed, when I press down on the gas pedal, it will make sort of a hissing grinding noise. It's usually after about 25 mph. I will barely tap and I will hear that noise while I am on the gas pedal then I will let off and the noise will sort of linger for a second or two. It doesn't make the noise when I'm in park but only when I am in motion. Any ideas on what it is? Thanks for the comments ahead of time!