Searched a good amount, didn't find a direct answer.

'05 MT 6spd
52K miles

I recently installed a genuine AEM cold air intake. Great results, pain in the *** to install! Here's the question: I've noticed a louding 'humming' type of sound coming from the engine bay, particularly when the car is idling and warmed up. It never occured with the stock intake on there. I'm not referring to the whistling noise that you typically hear from an intake when you open the throttle body valve i.e. rev the damn thing... This sound is almost electronic in nature, very steady and increases in noise level with engine temperature - you do not hear it when the engine is cold. And yes, I was sure to buy the appropriate vaccum line cap and cap off near the intake manifold, so no vaccum leak issues as far as I know. Is this a result of having removed the VFAD? I have not noticed any performance issues, but it would be foolish to wait for a problem to show up before assessing it. I'm assuming that the stock intake muffled this sound and now its very apparent with a cold air intake installed. It'd be helpful if anyone with an AEM CAI installed or even a cold air installation offer their input. Thanks!

 

Hum...i have the MS CAI (basically the same thing) installed...but I didn't cap it, have 3 tubes running to it, the section after MAF and before TB. Did not make any noise during idle. I've heard capping it could make noise tho. I'm gonna install oil catch can soon and cap the top one, we'll see...

 

Oh no no, I didnt cap the three 'major' lines that feed into the intake; the only line capped is the small one (1/8"??) that came from the front of the stock air box -- I did it as per the AEM install instructions. But you say you have no sound at idle??? Hmmm...

 

You might wanna post a clip, cuz' sound is something hard to describe, they tend to confuse people....
It could be nothing.
But I'm really curious about how you describe as "This sound is almost electronic in nature, very steady and increases in noise level with engine temperature"
Most intake noise only associate with rpm, from my limited knowledge.
Hopefully someone with much more experience would chime in...

 

Probably, something is not torqued enough that it's vibrating. Check the mounts and bolts.