I bought this truck just about a year ago and it had/has some mods on it including a custom air filter on the end of the stock air intake hose. When it is hot out, aprox 75 80deg + i get a very loud howling sound coming from inside the engine. The sound is only there when i am at a idle and not moving, once i start moving the howl goes away. First i took off the air filter to see if that was the problem, it wasn't so i disconnected the entire hose from the engine, the howl is coming from inside the air intake on the engine. (i don't know the name) the round flap that pivots in the center letting air in and out has 2 tiny holes in it on either side, I don't know if they are normal or not. I am trying to find out what could cause this sound or if something has been removed etc. thanks for the help in advance
-Mike a

Edit: I talked to a mechanic over the phone and he said it was for sure my cat converter, however a couple phone calls later to diff mechanics. many of them said it would have nothing to do with it. ty

 

It is your IAC pull it off and clean it the howling will go away

 

x2 on the IAC, since I replaced mine I have no howling even on those 100+ degree days. should have just cleaned it instead of replacing it but w/e

 

x3 on that.
and that part that you are talking about with the round flap would be the Throttle Body (TB). And yes, the holes are supposed to be there.

 

Awesome. Thanks everyone.