howl from under the hood - possibly with intake tract?

[Dazzlin' Dave]

Howling Noise

I had a howling noise in my 4.0, thought it was alternator bearings, not, then waterpump... not! Damn! After all that work, it turned out to be a bad Idler Pulley Bearing!
I replaced the WP because after a while of howling noise I blew a heater hose and the damn thing sprayed the back of the WP pulley!! I thought it was coming out the weep hole!! Tricky little Gremlins under my hood!!!!

 

[97-4.0XLT]

Was having same issue when my truck had warmed up (and especially with the A/C on), it would put out this loud (woooooing) mooing... actually it is more like a "fog horn" sound, but once I started moving again it quieted down..

Having read similar comments I went right to replacing the IAC, and did so and cleaned the throttle Body as well. But what happens on the first crank it starts making the same sound.. now it seems to make it intermittently even when under power and moving.

Do these things need time to break in?
What else could I be running into?

I am lead to believe the problem with my new IAC is that it does not have the "pressure valve" Black filter cap, like my stock one... It is just sealed off there.. Could this be the reason it is making the noise?

I put the old one back on and took off the filter cap... which removed the sound unless you put your finger over the pressure valve.
I was wondering does the filter cap do anything besides filter the air that the pressure sensor releases? Maybe I should just leave it off?

With the filter cap