Son-in-law's 2003 with 4.6 liter V8 and at idle and up to about 45 MPH there is a whistle from under the hood (not always present). This is not an electricl sound or from a pulley but a definite whistle. Any suggestions as to where too look, maybe purge valve?

 

Sorry, I couldn't resist...
http://www.youtube.com/watch?v=n79jSzKA0L8

But, if all hoses and lines are good, maybe a cracked fan shroud?

Does it do it with the hood up while revving the engine?

 

1. (i think its a typo) but he definitely does not have a 4.6l i think you meant 4.7??

2. where is the sound coming from? open the hood and listen, whistle is 90% of the time air movement over an unfriendly (to fluid flows) shape, most often it is caused by a vacuum line, however without an idea of where on the engine it is we are shooting in the dark.

 

OK 4.7 (Non-Hemi) and with hood open,the sound appears to be coming from passenger side and back near firewall, just cannot isolate it.

 

You could have a hole in the exhaust somewhere, try blocking the tailpipe a little to increase the back pressure and see if the sound changes.

 

4.7 was never offered as a hemi, thats the 5.7 (just so you know)

and passenger firewall is were the vacuum reservoir is on that truck (its actually under the wiper cowl), i would check for leaks in that, does your cruise control work correctly? (if you need the diagrams for removing and/or checking the reservoir let me know i can get you a link to just that section, or you can download the 2002 factory service manual from the FAQ thread, its close enough to the same that it will work for this project)