Did a little googling. Methane is primarily obtained from natural gas fields which are below ground. Atmospheric concentrations are relatively low (on order of parts per billion) although it is still an important greenhouse gas (30x more potent than CO2 per ton). There's a lot of ongoing research into generating CH4 from CO2 using renewable energy but it's relatively energy intensive to date.
So there’s noise pollution on a grand scale, it depends on natural gas extraction or a breakthrough in electrolysis, and it will produce CO2 when burned in flight. Great. It will also cost a fortune, and even SST isn’t viable, yet somehow this will be?