1999 Ford Escort

I have a whistling noise, it sounds like a pulley squealing. I can turn the car off, and it continues awhile after, fading off till its stops. The noise doesn't seem to go faster with acceleration, just get's lower. I believe it has too do with an air leak, but I'm not sure. I had a local mechanic look at it, he stated that it was exhaust related. I had both tentioners replaced so that's not it. The water Pump is new. I just had both Timing belts replaced. I am at a loss.I just can't narrow down exactly where it's coming from. If it is exhaust related, how do you find such a small leak? Please Help!