Loud virbrating exhaust when idle & AC is on

There is a very loud rattling sound that comes from the exhaust/muffler when my car is on and I stop (put the car in idle). It doesn’t happen every time, but if my AC is on, and I’ve just stopped the car at a light or stopped the car while parking/backing out of a parking space, the noise is very likely to occur (more than 50% of the time). The noise often temporarily goes away after 5 seconds or so when stopped. If I idle forward a few feet and hit the breaks again, the noise will usually start up again. The noise never occurs while accelerating.

The AC also seems to work well. Although I once had trouble getting up a mild hill with the pedal all the way floored.

Two mechanics have looked at the car. One did not find anything. The other found minor scorching in the spark plugs [hence explaining the hill issue], so he replaced those. But this did nothing to help with the noise.

2015 Suzuki Celerio, automatic transmission, with 30,000 miles on it. It has been serviced religiously every 6 months / 2,000 miles. 3-cylinder engine.

Did they check for a loose heat shield?

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Agree with Corollaguy. I’d look at the exhaust first for the rattling. Loose heat shield, clamp, broken mount, etc. Possibly the material inside the catalytic converter has broken loose and is rattling. If the rattling is in the exhaust system, you or your shop should be able to find the source of the sound by tapping down the length of the exhaust system with your fist (or a rubber mallet if you’re fancy). I have a rubber mallet now, so I am officially fancy.

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Yes, likely in the exhaust. The small change in RPM that occurs with AC compressor on/off can put the RPMs just where they set up a resonance with a loose exhaust part.