Then why continue with this on the 13" topic ?? and you did ask what the input cover is so people will respond to the 13" as in title etc.
If you’re only getting the rattling / buzzing / resonance on one side, consider yourself lucky.
I get it from left and right…at difference frequencies. 85Hz-ish, 160Hz-ish, and 440hz-ish on the left. The right side seems to be a anything between 100 to 380Hz-ish…almost as if multiple components resonating at different frequencies. Kinda like a left → right resonance crossover.
So is it the speaker itself is blown or rattling against its own mounts? Or is the speaker somewhat loose, touching the keyboard above it when the case is fully assembled and resonating against the keyboard?
Anything that resonates at the same frequency as the test tone or at a harmonic of it could be sympathetically vibrating. Wondering how much of a peak there is on a frequency response sweep at those frequencies.