if the leak is worse when the truck is cold, then it is probably your plenium or intake manifold gasket.
if you havent replaced already you should probably change it.
the throttle body gasket i wouldnt think would be a loud whistling sound. There isn't that much vacuum in there

 

its the same sound even if the truck is cold or warm. How would I know if the plenum intake is leaking? I just bought the truck from USA and have no idea if the plenum has been changed or not. How much does this cost? and where do I get the parts? and what parts do I need to change the plenum intake?

 

Huh ... when the butterflies open ... just about ALL the vacuum is there ... sucking the air through the filter. If that gasket is split it can absolutely cause a whistling sound.

Changing the plenum gasket (and belly pan if you're smart) isn't something you do until you need to. Excessive oil usage and pinging are the usual symptoms (though not always). If it's not causing a problem ... leave it alone.

http://www.hughesengines.com/partDet...p?partID=11698

 

Havent noticed any oil usage by now, but I just changed the engine oil this week so I'll check the levek again soon. My friends arrives back in Norway tomorrow with the new gasket so I'll get to change it this weekend!

 

i just replaced my stock air system with an airhat and my gasket was missing about half the circle. i didn't bother to change it, i hope thats not an extremely bad choice? sounds almost like i have a turbo under the hood ha.

 

Extra bad choice ... you're sucking unfiltered air where ever there's even the smallest gap between the TB and air cleaner base.

 

changed the gasket yesterday, but the noise is still there.. what else could it might be?

 

my moms 99 318 whistles too. when u going bout 40 45 with ur foot barely on the gas. when u take ur foot off the gasor more on it, it will stop. Also when you first start it up in the morning it would sound like u lettin air out of a tire or sumthing. like tshhhhhhh...

 

Not sure how to describe this sound, but its its not a tshhhh sound, its more like a whiiiiiii shound... Hmm.. this probably sounds stupid The sound comes around 1500 rpm, no matter even if I let go of the gas or keep it steady or accelerate.. But its not always that the sound comes...
And, whats the difference between hughes engine plenum kit and aps precision plenum kit?

 

Looks like the full Hughes kit is about $6 cheaper ... shipping dependent.

EDIT: So would you re-categorize your "whistle" to be more like a "squeal"?