I heard about that, but I find it very unlikely that they could make a noise loud enough on the carbon fiber hull, not exactly known for it’s acoustic properties and the resin acts as a sound damper. I’m not a material or acoustic engineer though, so who knows. Plus, the article did say that the sounds had stopped.
If it was actually them, I’d guess they were banging on the titanium end cap.
I feel like if there was a way to dampen sounds that well then military submarines would have adopted it long ago. No idea though.