'Barrelling down' usually means 'to move with great force'. In your example, 'barrelling down' is used in a more figurative sense. The noise generated by the stands was very, very loud. So loud that it seemed the noise had been forced onto the pitch and felt like a physical presence. The noise is so loud that it is being practically thrown at the pitch. It is 'barrelling down'. Unrelated to the question, but I've heard 'barrelling down' used in reference to someone staring at someone else.

I believe that here it means "moving at high speed". It is describing the voices of football fans shouting and cheering being projected towards the football pitch. I think the meaning comes from the image of barrels rolling quickly down a hill in a somewhat relentless and uncontrollable fashion.