Hard to say:
1. Different cases have different acoustics. Some cases sound louder given the same fans running at the same speed as in a different case.
2. How the fans are mounted makes a difference. If som kind of insulating grommet is used instead of having direct fan-to-case contact sometimes makes for a quieter case.
3. If the airflow in the new case is considerably worse than before, your system may just be running hotter, leading to higher fan speeds most of the time.
4. If motherboard is the same, and it has any kind of fan management feature, you may need to re-run whatever fan setup BIOS has for the new case.
5. BIOS and/or fan utility settings have been reset and everything is running full speed regardless of temperature.