Engine Sound Change 30 - 60 Seconds After Engine Start

'14 Access Cab, Base, 2.7L, Auto, 5-Lug, 30K miles: so 30 to 60 seconds after starting the engine from cold...I hear this very brief (0.5 seconds ish) buzz sound, and the engine running "tone" changes. It's as if something has turned ON or OFF...maybe after the engine has reached some temperature? Anyone else hear the same?

 

There's an air pump that's connected to the exhaust header (you can see the pump it's near the righthand headlight in the engine compartment). It feeds air into the exhaust for a bit after engine start to help the catalytic converter do..something. I honestly can't remember what it does. I'm sure it's important. Anyway it's normal.