Sound like the air compressor kicking off when cold

I have a 2006, 4.7, 4x4 with about 80K on it. Only when the engine is cold it does this, but a short time after startup, the engine lets out this sound as if the air compressor is kicking off and it can breathe again. It's not the air compressor (I don't think). After that, Rex purrs like a kitten. I'm not extremely concerned about it, but I would like to know if it's normal. I don't remember it doing this before. With age, maybe there are vacuum issues? I really just want to get ahead of any potential problems.

 

It’s called the secondary air injection pump system it’s normal, and it’s normal for them to fail

 

Thanks for the info. At least I know it is a thing now. What do you mean by fail?

 

There’s valves and pumps and sensors, not sure which is the most common failure, but little worry, there is a bypass for certain failure codes? Hewitbypass or the likes, is one of them.

 

Got it. Thanks a ton!

 

It’s a emissions thing. That’s all. It helps pre warm your cats.

 

If you run the engine for 10 or 15 minutes, shut down, then restart immediately and don't hear it, highly likely it's air pump as it wont activate when the cats are hot. If not, could be a serpentine belt.

 

That describes it perfectly.

 

Are you throwing any codes yet (check engine light) for it?

Pump is expensive, but can be bypassed, though not technically emissions-legal.