hsss noise '03 Mountaineer

[ashcroft]

My mountaineer makes a hissing noise that occurs when ever I accelerate. It is usally at speeds above 45 miles (give or take). I've had it worked on two different occasions. The first time they found nothing. The second time, the couldn't find the problem...however, they had to clean a bunch of carbon (I think) out. I'm not sure if that was related to the problem or not. The noise is still there and I can't find anyone that seems to be able to tell me what the problem is....any suggestions?

thanks