Each unit contains a temperature dependent cooling fan. This means the hotter the work environment, the louder the fan. Higher sample rates equal hotter units and thus, louder fans.
Units mounted in an open or air cooled environment will put out less fan noise, whereas units in sealed racks will produce more noise. The fan sucks air across the unit from right to left (when viewed from the front), so if possible, keep an area clear on the interface's right hand side.