Whistle from Truck?

Hi, I have a 1998 Cummins diesel. After shutting off the truck, I get out and hear this whistle coming from somewhere in the motor. How I get rid
of it is I loosen the fuel cap and I hear a rush of air (tank had a vacum) and the sound goes away. Is this a faulty fuel cap? Should the cap vent the tank somehow? Thoughts?... Thanks...Jim
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