Why does Rinzler (Tron) purr. Is it because of the damage that he sustained when Clu attacked him?
The only potential clue comes when Tron falls into the sea after betraying CLU (around 1:05 in the clip below). The sound is very distinct when he passes the camera and then stops with the color change from red(CLU) to white(user)
I'd say that the purring at the very least represents the control CLU has over him.