"Try using the balance control in the sound control panel to isolate what speaker is having problems. Don't forget you also have a third speaker! A subwoofer.
It's possible you have something loose which is vibrating. Have you had anything altered in your system? If you did I would review that area."
"I have replaced both speakers of my Mac Pro A1398 Mid 2015, on three (3) times. Still it keeps on bursting the edge of the speaker - rattling sound.
I THINK APPLE SPEAKERS ARE DEFECTIVE FROM THE TIME OF MANUFACTURE.
I have ordered another set this time from Ali express. Last replaced in December 2017. Last pair bought from ifixit, it lasted for 6 months - longest period it lasted. Now again rattling."
i have a same problem does apple fixes for free ? it its been three years that i have this mac, which is already over the warrenty preriod . my one is 15 inch mac book pro retina late 2013.