Sound under high-speed/constant RPM

Hi all.
I'm brand new to these forums and would like to ask some opinions.
I have a 2001 2500HD 6.0L with 18,260 miles..Normally this truck is a honey, the best truck I've ever owned and I treat her right and also don't drive her much.
But under highway speeds, say 60-70mph, at a constant cruising RPM, I hear a sound from under the hood about every 20 seconds or so. It's something to do with the throttle, I have deduced, because you hear the same sound when you put her in the corner a little bit, like the throttle body opening up and sucking air in. Except, I am not accelerating quickly, I am not pressing the pedal any further. I'm not seeing any loss in power, the truck isn't throwing any codes, and you can't feel anything within the truck. It's just really, really annoying. Any help anyone can give me is greatly appreciated. Thanks alot.

Could be droning from the exhaust maybe. I know my truck did it the most around that speed. It also seemed to vibrate a bit b/w 65-70.