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A riddle for you guys. I've a 2015 z51. I hear a brief chirp / whistle / squeal / shriek sound noise when I accelerate. I happens briefly at around 1500 rpm while in automatic. Any mode. Eco, touring, sport, or track. The noise never happens when I'm in manual. What could it be?
 

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Welcome to our forum. Sounds like it might be the exhaust flaps.
 

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Could be the 4 banger flap. When my M7 went into eco mode (4 cylinder) there would be a hiss/whistle sound from under the seats. Maybe yours are stuck closed or partially closed.
 

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That's just how I refer to them. When you go into "eco" mode, you are deactivating 4 cylinders when cruising. If you accelerate, the 4 will reactivate to give you the power you request, then deactivate again when back to cruising. Because you effectively reduce the engine displacement from 6.2 to 3.1 liters, the exhaust parameters need to be different. To get to the correct parameters, there are a pair of butterfly valves, similar to the NPP valves, to increase the back pressure for the smaller displacement. These butterfly valves are perforated and, I believe located under the seats. When mine went into eco mode and deactivated the 4 cylinders I could hear the whistling or hissing from these closed butterfly valves.

I'm just thinking you could compare the position of the eco actuator arms to those of the NPP actuators. The actuators are probably identical to save production costs so I would imagine they would be in the same position when open and closed. The eco mode butterfly valves should only be closed when "ECO Mode" is selected and when cruising. Yours may be stuck closed.

Do a search on Eco Mode and Active Fuel Management (AFM) to learn more.
 
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