Download an audio frequency generator app to your smartphone.
Then manually sweep the output from the lowest frequency tone to the highest audible frequency tone (about 20,000 cycles per second), while playing the signal through the car's stereo. Somewhere between the lowest frequency and the highest frequency you are likely to notice the offending noise to be most apparent. Then while keeping the frequency set at that point and the offending noise being rather constant start poking/pressing/feeling around the various surfaces within the car's interior to home in on the source of the noise.
The app I use is called Signal Generator (I think), and the basic app is free (I think).
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