Weird whistling sound
 

I know I know, sounds are quite hard to diagnose without hearing them, but this one is pretty obvious. I might take it into the dealer to have a listen but I'd figure I'd ask here just in case anyone knows what I'm talking about.

Ok, I give my truck a little bit of throttle and, as the engine is revving down it makes a whistling noise, I'll be more specific- the noise sounds really quite similar to crickets chirping, except the whistling is constant, rather than broken as a real cricket's chirping would be. As the truck drops back to idle, the sound ceases.

I only ask this because my truck seems to be the only one that does it. My Dads 01 5.3 Z71 doesn't make the whistling noise nor does my brothers truck.

Anyone heard anything similar? I thought belts, but its not really a squalling sound like belts make. I'm thinking its something with the intake but I really don't know.

You're not using a TB spacer are you?

Make sure you intake tube is sealed aroung the bottom of the T/B, and everywhere else for that matter.

I have to ask the same question, do you have a TB Spacers or any intakes mods? I picked up a whistle when I did my K&N FIPK II and Spacer install. Replaced the spacer with a BBK TB a bit later and still have it (The BBK has a about a 1/4" spacer to reducer it back down to stock intake diameter :( )
Whistle is still there for me, but I don't mind it much...

No, I don't have a TB Spacer. And I just got done messing around with the truck, the intake tube is definitely sealed to the TB.

I wish I didn't have this PITA electronic Throttle Body, I have to have a friend tap the gas to hear the whistle, like I said it makes the noise as the engine slows back down.

It almost sounds like the noise is coming from somewhere within the intake manifold. Lemme ask this, if the intake manifold were leaking would the engine probably throw a code?

Yeah, I have that same whistle too, I know what you are talking about. I hear mine only when I'm driving, it's not there when reving in park or neutral. I have a K&N FIPK and the free mods. I think it is caused by the Mass Airflow Screen Removal. I remember it starting right after I did that mod. I can hardly hear it when I take off, but I think that is the exhaust because I could hear it better before. No leaks anywhere in the intake though, also no service engine lights and the mass air sensor still looks fine. It has been this way for a few months but I really like it. It happens when I let off the throttle, under WOT sometimes, and right after I shift gears, almost like a blow-off valve sound. I personally think it sounds pretty cool, and even have friends ask if I have a supercharger or something. I smoked this lightly modded manual Escort Sport ZX2 that a coworker and friend drives a few days ago and the 5 people in his car wouldnt believe that I didn't have a supercharger until I actually opened the hood and showed them. They said it had a wicked sounding whistle when I hit it and then I was gone.