When I am ideling in PARK, every 3-5 seconds it feels like the RPM's are dropping just a very small bit. I can't see the RPM guage dropping, but it I will notice a small vibration lasting only about a second and then goes away. When I am in DRIVE or REVERSE, it doens't do it. I didn't know if this is something normal that I am just now noticing or if there could be something else going on. Any ideas? The truck runs strong with no hesitation or anything. It's a 2004 4.8 liter. Thanks