Never experienced this in my MX which is on 2018.42.2 last I drove it; it's in service center right now. However, I'm currently driving a 2018 MX 75D as the service car, and it has had this issues a few time.
I noticed it first happened when I was switching from using music via Bluetooth from my iPhone 7plus with iOS 12.1 onto the Tesla streaming music. I shut off the Bluetooth radio on my iPhone; the static didn't go away, however, when I hit the pause [II] button then hit [|>] play button again, the static went away. Reminded me of the old days when you were about to get a phone call, and before the phone rang, your computer speakers would start to make weird static sounds.