I began hearing this noise when as soon as i installed my MD 1/2 Copper iso-thermal spacer. Here's what I think, and what i did:

1). If you have the copper kit, you replace the gasket in the throttle body with the new copper gasket and connect the rubber tubes to this, but, there's still the old tubes coming of the throttle body that don't connect to anything. I *think* that there is where the whistle is coming from. I put electrical tape over the tubes that didn't connect to anything that did seem to quiet the whistle. Its not totally gone, but its significantly less.

2). The whistle will also come from the extra volume of air now being sucked in by the intake.

I don't know if #1 made sense to you, I prolly didn't explain it as well as i could, but its early, and i haven't had coffee yet

 

Update, the whistling sound went away after i retorqued the bolts of the 5/16 spacer. Didn't notice any leaks though.

I installed an intake(Blitz Sonic Power) today and still no whistle. I miss that whistle lol.

 

Mine is stock (as far as I know) and I get a whistle around there too...

 

I have the sucking sound but no whistling.

best description of the sucking sound would be the sound sucking in air with your mouth lol.

the whistling sound I had sounded like the turbine of an aeroplane.. but not as loud.

 

Mine whistles like a toothless hillbilly with a bad lisp.

But I have no vacuum leaks.

 

Isnt that coming from the spacer?

Someone correct me if I am wrong.

Is this what it sounds like