SAI (secondary air injection) is a emissions thing.
The catalytic converters need to be up to temp to work, to speed them up getting to temp they force fresh air into them buy using a air pump.
It works like a blacksmiths bellows, he blows on his fire and it gets bigger and hotter to heat the steel he is working with.
The SAI on your truck does the same thing.
It gets noisy when the fitler to it gets clogged, it gets bad enough and it will stop working.
In the "how to" section there is a wrte up on how to replace the air filter to the SAI.
SAI is a good thing.

 

Hi, i'm new here and just wondering if anybody has a sound like this? I've been doing a lot of searching and this is probably the most similiar. It only happens after a long trip. I only hear it when in drive and stopped. As soon as I put it in park or drive away the sound goes away. I feel like it maybe rpm related since the rpms are lowest in gear and stopped. I'm thinking torque converter, power steering pump, or transmition. Any thoughts?