Hissing at acceleration - exhaust leak?

My Chev Impala 2001 base (57K) has been making hissing noise at acceleration. The noise came into the cabin from front somewhere down under the hood. Got it checked by dealers/mechanics for 3 times. They all say "no
problem." or "normal". I know they must have checked cat convert and vacuum leak etc. But the hissing has been louder and louder when accelerating even at very low speed. Another symptom is poor acceleration and a little worse gas mileage together with the hissing noise coming up a few months ago.
I'm just wondering if this could possibly be due to a leak somewhere in the emission system other than a plugged catalytic converter? a loose joint in exhaust? The noise does sound like air/water spraying out from a high pressure pipe. Would it be wise to get the car back to dealer again, and just ask for a check on exhaust?
Thanks!
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An exhaust leak usually has a rumbling sound. You will likely be able to hear it even at idle. A hissing noise doesn't seem right for a leak...
Just because you took the car to a mechanic, it doesnt necessarily mean that he checked anything at all. Maybe just gave you an opinion.
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An exception to this is a hole in the tailpipe past the muffler. My father had a piece of rotted tailpipe blow out ounder hard acceleration on a Suburban and it hissed/swished on hard accelration. Roy

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the wp bearing is about to die.
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Dave wrote:

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