Kung nasa gulong or brakes, usually consistent yung sound even if he was coasting in neutral, AFAIK.
Direct ignition na yung IDSI engines so no need to adjust timing (wala nang distributor na magagalaw). Next is to load up on the next grade fuel and check your spark plugs. Try also another gas station and fuel brand. In my case, my friend's 2011 City 1.3L was getting a bad case of tope using Petron unleaded (91 octane); she she switched to Shell Fuel Save and the tope went away. I also had the same problem with my Altis on Petron XCS, may tope na lumalabas in the last few months under certain driving conditions. I'm now using Shell Nitro+ and the tope issue went away.