When I flush the toilet or use the sink I hear hissing in the toilet and gurgling in the bathtub. The house is 2-story and only 3 years old. What is the problem and how can I fix it.

I think your vent stack may be partially blocked. The gurgle is your tub trap trying to act like a vent. Rod from the roof with a sewer rodder long enough to reach roof top to basement plus 20'.