exhaust noise "at times"

Loud exhaust noise in my 2005 LeSabre, once in a while. Exhaust system gone over by mechanic, nothing found. Runs quiet for several days and then, engine warm or cold, sounds like a truck without exhaust system. Noise goes away after driving. Help!

Are you sure it's not your cooling fans kicking in?

In addition to what the "kat" said, possible air pump?

Thanks for the tip. My mechanic and I have checked just about everything. I recorded the sound and it is like an exhaust bypass sound. You would think that the exhaust pipe had come loose. The whole exhaust system has been checked. We have had a lot of ice and I am beginning to think that maybe some ice gets up under the car and pushes things around. My wife is driving the car today and it sounds like a new one. Guess it might have turned into a warm weather car. We got it new and never had any trouble. Thanks to all for the tips.

how does the car run ? gas mileage ? anything odd like oil in the coolant, coolant in the oil ( mayo looking stuff on the dipstick) any oil leaks. Is the pavement dry and spotless under the car after being parked all night? can you predict when this noise will happen ? has your mechanic actually heard the noise. Sometimes one mans noise sounds totally different to a mechanic than your description. Why not let your mechanic drive your car for a few days and you drive his or a rental. Could be an EGR or an exhaust gasket. Take the car to another mechanic.

Thanks for the tips. Good gas milage, upper 20's to mid 30's. Coolant clean, oil clean, the car is running like a new one right now. The noise happened two times. Once to my wife and once after church to me. The car sat outside in below zero weather and we had just passed through some slush. Church lasted a little over an hour. When I started the car I just knew that the exhaust pipe had come loose. By the time we made it home, about one mile, the noise stopped and has not returned.???? Like I say, guess it has turned into a warm weather car. Just broke in....150K miles. Thanks to all.