My 2019 Honda civic makes rumbling noise on occasion not all the time,

What is your question?

I have had the mechanic at Honda look and drive it and of course never happens then. I am not sure if this is a transmission issue, tire (tread?) or bearing issue. car rides smooth but the noise is bothersome and worrisome? It does not happen all the time, I just fear being broken down.

How long has your 2019 Honda Civic had this problem?

6 months