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Loud vibration sound

I have a 2004 outback with just over 100k miles on it. It has been making a loud vibrating sound lately. It makes the sound when idling and even louder when I step on the gas, but at highway speeds the sound disappears. I do not actually FEEL a vibration, it just sounds as though something is vibrating/rattling. We had a mechanic check it out and they thought it was an exhaust issue. Apparently not, because the sound is still there.

You didn’t tell us where the sound is emanating from, but if it seems like it is coming from underneath the car, the most likely explanation is a loose or bent heat shield on the exhaust system.

“We had a mechanic check it out and they thought it was an exhaust issue. Apparently not, because the sound is still there.”

Does this mean the mechanic changed something with the exhaust?

Let’s start with locating it. Get about four feet of garden hose and use that as a stethoscope and locate the source of the noise.

Sounds like the exhaust manifold heat shields have come loose. Happens frequently. First time I had them replaced at the dealer, very expensive. Second time I tied upper and lower shields together with a heavy gauge wire twist tie. Just drilled a hole in the standing seam of the lower shield and connected to an existing hole in the upper shield. No rattles since.