I just bought my first diesel a few months ago and I'm still trying to figure this stuff out. I have an 01 7.3 with the 6spd transmission. The only mod to my knowlwdge is a k&n coldair intake. When ever the truck is under load going uphill (especially while towing a 7000lb camper) i hear a "ssshhhh" type of sound. It always happens between 1500-2000 rpm. As soon as I get to the top of the hill and the road straightens out, the "sssshhh" noise goes away even thought I maintain the same rpm. Is this the sound of the vehicle sucking air throught the intake or do I have a leak or some other problem? I tightened all the boot clamps and the sound is still there. It's driving me nuts because I'm concerned there is something wrong witht eh truck. I would appreciate any input, and thank you in advance for your suggestion.