06 4.0 5spd with 54,000 miles. OK Ive been trying to figure out what this is but for some reason my truck has like a slight squeel. not like your normal belt one but the best way I can describe it is like a high pitched air pressure sound. like i can hear the air pressure sound from inside the cab. it doesnt do it when standing still but when I go through to gears it does it. It does it driving in like 1st, 2nd, and 5th?? Like it will make the sound when driving in 5th but if I take it down to 4th magically gone, yea weird I know, and if I put it in neutral it goes away. but like i said only those gears and I can hear pressure changing in the dash area. what you think IAC valve, EGR valve or what. I dont think its anything wrong with tranny because its not every gear, its not when standing still or off take off.

 

come on guys, I know as many people on here that have a TON of knowledge, that someone could say something about it

 

Yea i have the same thing. usually when i hit higher RPM's though, i was thinking it may be a vacum pump? ive always wondered abt it myself too man