1. Try pulling your PCV valve and shake it , if it sounds like an empty paint can then it's good - if questionable , just replace it - it's only 3-6 bucks and this has been known to cause this problem.
2. Sounds air/vac related. Start your truck, let it run for 30 seconds, have your hood open before you start it. Shut the truck off and go look under the hood quickly - do you here a noise like air releasing?
3. Check the vac line behind the manifold at the 90 - this has been known to collaps/leak and could be making the noise at a certain RPM's.
Heres a pic of that line , the 90 by the firewall is the one that I'm talking about -