I own a 2005 Trailblazer that makes an awful whining noise whenever it is on, it gets louder as I accelerate but never goes away and the check engine light is on. I was told by a mechanic that it was either the secondary emissions pump or the relay switch that controls it. From reading other information, it seems that the noise would go away after starting the car if it truly was the pump. Can anyone clarify this? Or have any idea of what else it may be? Thank you!

 

Mine whined and it was an Idler pulley. $20 maybe the CEL isn't related to the noise.

 

I had an 03 that did that and it was the thermostatic fan. It was replaced and the noise stopped for a while but came back.