matrix starter problem?

my 2003 toyota matrix , when starting, makes a strange kind of fluttering sound from the engine compartment, then turns over slowly as if the battery was nearly out then starts up and runs fine. it does it when sat for a while, hour or 2 not minutes. any ideas? the battery indicator on the actual battery is green- for ok and no warning lights are on.

go to a parts store and get a free battery test. id bet you need a new one.