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2012 KIA Optima 2.4L (EX)

I noticed there's a metallic sound from the engine (like a buzzing sound) when applying light pressure to the gas pedal to accelerate, it's really only noticeable at speeds below 50MPH. When I let off the gas pedal, the noise immediately goes away. I have eco on at all times. The engine (2.4L) is original and has about 124,000 miles. The noise started in the last 3 months or so. The car has no error codes stored in the computer or illuminated warning lights on the dashboard. The car is getting slightly lower than average MPG for the winter, but it's not terrible (~30 MPG, mostly highway). I typically hear the noise when driving around parking lots, when I'm traveling at 25MPH or lower. It only happens when driving forward. I don't press down hard on the gas pedal and typically use light acceleration when possible to get up to speed unless someone is behind me or I'm merging into traffic.

I had one shop look at the issue today (not a dealership) and they noted that the exhaust shields are all tight and that the noise is likely from noisy GDI fuel injectors. Two technicians listened to the noise and came to that conclusion. They believe the noise is normal and suggested changing fuel brand and/or trying BG Ethanol Fuel System Defender, which is a 2 part cleaning system which removes allegedly deposits and water. I typically run Techron every other oil change and I usually buy my gas from a Shell (top tier) gas station down the street from the shop. The owner of the shop said they've had many cars towed into the shop after buying water-laced fuel from this particular Shell gas station.

What do folks think of this noisy injectors diagnosis? Assuming the issue is related noisy injectors, any thoughts to make them quieter?
 
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