Hissing and power loss
 

I have an 03 galant today wine driving down the road I started to hear a hissing sound from under the Hood and I lost acceleration power it still drives but takes a wile to get up to speed and the hissing g kicks in around 2.5 and 3000 rpms a d as soon as it starts the acceleration drops a lot if I floor it I get no change at all I've checked hoses and they all seem fine dose anyone have any idea what it could be