help, pls.

our car has been having this muffler-like sputtering sound whenever i'm pressing the accelerator. this occurs especially from cold start and at the lower gears (1st and 2nd). really irritating. when the engine heats up and has sped up then the sound minimizes but does not completely go away.

any ideas what the problem is? my feeling is there's a problem in the combustion. spark plug problem? gas sensor problem?

btw, i think that my gas consumption increase since the problem occurred. not sure yet though. will observe further.

tia.