The Octopre will get very hot when it is on since it uses class A circuitry - do not worry - this is normal!! There is a cooling fan in the Octopre whose speed is governed by a thermistor so the hotter the unit gets the faster the fan will spin. There is a ventilation hole on the side of the unit and if this is being covered somewhat then the airflow may be restricted and the fan will run faster. We offer no options for replacing the fan or using a heat sink instead but if fan noise is a problem then the first thing to check is that the unit has enough space around it for heat to dissipate and that the vent is not being blocked. If the unit is in a rack then we reccommend leaving 1U space above it to prevent excessive heat building up. It is also worth considering where the Octopre is placed in the studio - moving it further away from the listening position will help.
It is possible to operate the unit with the cover removed so that heat dissipates and the fan does not spin so fast. Owing to manufacturing laws we could not put any ventilation holes in the top of the unit but it is not unreasonable to do so yourself. Focusrite do not endorse these suggestions but if you choose to try them out then make sure that the unit is not at risk of having anyhting fall into it and also ensure that the earth wire is NOT disconnected from the lid.