Computer fans are necessary because they keep the internals of your computer cool, preventing overheating and system failures, but they can be noisy. If a fan does not have a built-in method for modulating speed, you can regulate the fan speed with a fan speed controller. This is a simple device that sits between the power supply and the fan to control the power going to the fan. Reducing the power will lower the fan's speed and can help to quiet your Dell computer.
Buy a fan speed controller from an online retailer such as Amazon or through an electronics store.
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Shut your computer down and unplug it from the wall. Remove the power cable from the back of the computer, then flip the on/off switch for the power supply, which is near the top of the rear panel of your case on Dell computers.
Consult your Dell manual to determine the proper method for removing the side of your case. Unlike traditional cases, which use a two-screw attachment method to secure the right (removable) side of the case, many Dell computers use a one-screw method, located in the middle of the case along the right-hand side or the rear panel. Other Dell cases utilize a no-screw attachment method, where the side of the case can be removed by pinching a switch at the top and the bottom of the case.
Disconnect the fan's power connector and plug it directly into the back of the fan speed controller. Dell computers are typically designed with all of the power leads bundled and tied together (typically with a simple twist-tie) out of the way, so you might need to untie them first to determine which is the appropriate lead for the fan connector.
Attach the lead from the fan speed controller to the slot for your fan's power, creating a bridge between the two.
Adjust the fan speed as desired by modulating the knob or dial on the front of the fan speed controller.