I had a rattle in my Model 3. I used this app to generate increasing higher frequencies in my speaker system (volume need to be high at low frequencies - gain is low):
Sonic is the original app used to clean water from your iPhone speakers. Sonic generates a sine wave tone with easy precise control over the frequency. Simply swipe your finger up and down anywhere on the screen to raise or lower the tone frequency. Swipe slowly for small frequency adjustments...
apps.apple.com
I was able to hit a resonate frequency that excited the "rattle" and I was able to find it easily. They weren't able to fully fix it, but it helped.