Thanks for the help. I have been dealing with this noise since August but only really applied myself to solving it in the past few weeks. This is what I found and solved it. Hope this helps you beat flat rate too!
Something is grounding out and finding out what it is can be time intensive. Can you disturb the noise if you put your hand on any part of the dash or A pillar area? If you, gently now, open handed hit your hand against the trim around the cluster, does anything change? I've had very little in the way of interior noise like that on Gen 7 Malibu. Only takes a minute to check the stuff I mentioned.
Now, only if you can do this safely, it takes two people. Put the parking brake on, and hold your foot on the brake, in drive, and get the engine speed to where you hear the noise. With someone under the hood and not in front of the car, see if they can disturb the noise by gently moving things around. Look for brake lines or cables contacting something like the overflow tank for example. Don't keep the RPM's up for long periods of time. 20 to 30 seconds, then let it idle for a couple minutes to cool down. Trans fluid temperature will rise rapidly doing this.
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