No need to worry bro. I am guessing that its just a knocking sound with absolutely no vibration or jerks. Because that is what I have experienced on my 2 Lancers. The previous one with a 4G13 engine used to do it and then I started mixing about Rs 500 worth of hi octane after every couple of tanks and the knocking sound vanished. My current one with a 4G18 engine did exactly the same thing and now I have started mixing about 200-300 worth of hi octane with every tank and the knocking sound has again vanished. I am, of course, assuming that you are taking good care of your engine and that it is performing fine, aside from this little niggle.