There might be a bit of misalignment of the cable running through the inline. I doubt it's at the main lever since the inline wouldn't work backward and make it squeak. Take a close look at the area near the inline lever or hold it as you work the main lever, and you might see or feel what's happening.
The solution might be as simple as putting a drop of oil where the wire runs through the inline lever (both the lever and body), or you might need to run some down the cable.
If the bike is new, let it be the dealers problem.
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