Loud horn sound coming from engine are.
My car is making a horn like noise by the engine when
idle, once i accelerate the noise goes away.
your alternator belt is letting you know that it needs to be changed very soon sometimes they call it the serpentine belt..
If it whistles continuously except upon acceleration, it is a the EGR valve. I had this problem on my VW Passat (2006) and there was a recall for the plastic part that failed. Upon opening the hood, you will know if it is a belt or a whistling relating to the intake. Belts squeal, and the weird EGR valves that fail make the most annoying sound in the world.