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!