It's not a rumor, SpaceX has stated publicly that they're working on LOX/Methane engines. There are two big reasons for this. On the one hand, it's just a good

It's not a rumor, SpaceX has stated publicly that they're working on LOX/Methane engines. There are two big reasons for this. On the one hand, it's just a good idea, liquid methane is a pretty decent compromise between LH2 and kerosene in terms of Isp vs. density (both of which affect overall stage performance) and since LCH4 exists at a similar temperature to LOX it doesn't add any exceptional problems into vehicle design or operations the way LH2 does. But perhaps more profoundly, methane is actually shockingly easy to manufacture on Mars with a very small amount of industrial equipment (we're talking about mere kilograms, provided there's sufficient power available). So the development of methane fueled rockets is also very much a way to build the systems and technology necessary for facilitating exploration and colonization of Mars.