piston movement is balanced, it's not like 4 cyl engine, that 2 are reaching cyl top and 2 are hitting bottom, they lag 60 degrees instead of 180. piston movement is imbalanced if one cyl is not functioning and crankshaft is pushing its piston instead of piston/connecting rod doing this to crank.
as far as noise is concerned, i think it depends on material used and compression ratios, and of course valves play an important role. cultus have only 6 valves.