funny noice when decelerating @ 70 MPH

[LIL' MIKE]

Took my truck for a fairly long drive (1.5hr) and stayed between 70-80 MPH. RPMs stayed around 1800 at cruise. Made a quick stop let it ideal for around 20 min then headed back home. On the way there i had the radio up and didnt notice anything different. On the way back someone changed lanes in front of me and i turned off the cruise and notice a high pitched vibration kinda sound which i also felt sightly in the floor board. It only last for the first sec or so. I got off at the next exit and didnt see any thing at all, but i noticed a "cam" like sound coming from the air breather (not sure if that was there before or not). At lower speeds around town and up to 65 MPH i dont get this vibration/sound, but now i get it everytime i get on the Interstate.