4.6l in crown vic making growling noise.

[FordGuy1085]

my sister braught her car to me because it is making this growling noise from the intake, i've been told to change the IAC. i just changed it a couple months back because the car was idling realy high but we all know how great aftermarket parts are. So just for grins i bought another aftermarket hoping i could pin point it, it didnt help, as soon as the car starts the growling is there, quiets when i rev it up but if i hold it about 1500 rpms it comes back. I've also been told that aftermarket IAC wont hold up and may not work at all and that a genuine FORD IAC is the only way to go so before i spend the $120 on this IAC im just wondering is there any other tests i could perform or anything else that could cause this symptom? Any feed back would be much appreciated, i'm a diesel mechanic, these newer gas engines got me stumped.