loud "sucking" noise from intake
 

My S4 NA has a loud hissing-sucking-howling noise coming from the intake. It is not just the normal air being drawn in. The block has about 20k miles on a Mazda rebuild. The noise doesn't start until about 3500-3600 rpm and continues up to at least 6 k. The motor runs pretty quiet until the sucking noise starts.

I took the air filter out and the noise is actually coming from the intake and it is loud enough to easily hear while sitting inside with the windows closed.

It also cuts-out terribly in the same rpm range.

Do you have the silencer connected to the ACV outlet? 3500 rpm is where air is dumped instead of pumped into the exhaust ports / cat. You'll be able to hear both noise from the airpump and the sound of air escaping. It can sound similar to what you described.