Start Softail FXSTB 2009 – Climbing at 40 km/h

Hello, everybody, I have a question. Have a brand new FXSTB added to me, which I’m driving in. If you hold the gas in the second gear at 40 km/h, you hear such a clump, about it and underneath it disappears. What do you think that could be? Is that normal?` Many greetings tch