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I posted an inquiry here several months ago regarding the rattling noise from under the hood when I put brake while I was driving [gear in "D" position; noise disappears in "P"], all after I changed the brake. The rattling noise is like a helicopter nearby.

Initially I suspected the brake because the noise started since I changed brakes, so I took apart the brakes, checked the anti-rattle clips and calipers, but nothing was wrong.
I searched online and someone suggested the heat shield or bad alternator as a possible source of the rattling noise, but mine was alright.

I brought my car to a mechanic and he used a stethoscope and listened to the engine and other areas. He blamed the intake manifold, but told me to live with it because it costs too much to fix.
The noise got worse over the months. Turning on the A/C reduced the noise a little.
It was a mystery and I was wondering where the noise comes from.

Recently I noticed a slight coolant leak from under the radiator cap. I didn't see the actual coolant, but white stains on the top of the radiator. It got worse. It was not leaking from the radiator it self, but between the cap and the neck.
I remember that I also changed the radiator around the time I changed the brake.
The radiator was not Hyundai OEM part, but after-market part from O'Riley's.
I had a life-time warranty for the radiator, so I returned it to the store and changed with a new one.

After I installed a new radiator, surprisingly the mysterious rattling noise was gone for good! What I noticed was that when I replaced a new one this time, it dropped down to the seat well and I was able to secure the top retaining screws easily. Last time when I changed it, it didn't drop down properly and I had hard time to tighten the screws to the frame.

My conclusion is that the radiator was not properly seated, nor secured. The circulating coolant caused it to rattle. Problem solved!
Tips: when you replace a radiator, replace the cap also to make sure it fits tightly. You can avoid changing a radiator twice due to defective radiator.
 
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