In the original Ramayana, Ravana is said to have 10 heads. 10 heads mean 10 brains, and that further means the possibility of having 10 different thoughts simultaneously.
In the movie, the character Veera is portrayed like Ravana, which is also the movie's name. Quite often when he thinks about something or tries to arrive at a decision, his 10 brains start transmitting ideas simultaneously and hence the collision of ideas and hence the sound "dan...dan...dan...danaka..." which could be translated in English as "clang.. clang..." That's actually the very essence of this character.
That's actually the very essence of this character.
Now the more important part. What is the source here or why should you trust this answer?
For the mythological part, you can check this. For the movie related part my answer is based on one interview of actor Amitabh Bachhan, who is the father of actor Abhishek bachhan, who played the part of Veera in the Hindi version. In the interview Amitabh Bachhan expressed disappointment over the editing of the Hindi version where this particular part was trimmed down. The actor explained why this part was important and how it's editing damaged the movie.
The link to which is here.