I know its like a rattle sound that comes and goes.Those are the valve lifters, it's common japan made cars have this sound with more than 100,000 miles. you can try checking if the oil is not too old, or if level is correct, if that is ok, you can get at the Autozone an "oil detergent" like Gunk's "CD2", if the sound doesn't dissappear, you will need to change it, it is about 150 Usd (parts and instalation).