MacBook Pro 16" Speakers Spontaneously Making Strange Noise. How do I fix this?
Can you report there as well https://discussions.apple.com/thread/251560058?
You are not the only one....
My guess is that it is not the sound card as headphones work fine, but could be a driver / compatibility issue with the speakers. Def looks like a driver/OS not working as expected to me
My issue happened with the exact same version: MacOS Catalina 10.15.6