The cost of air compressor repair Holly generally depends on the damage to be repaired and the materials that need to be used. Some people who have the background and enough knowledge of air compressor systems repair their units on their own to save some cost. However, if you do not have the know-hows, you might as well hire a professional to do it for you or you might risk causing more damages. The most common issue that may need Holly air compressor repair is when you hear a humming sound or noise from your compressor. If that is the case, the issue may be caused by a bum unloader valve. Unloader valves are easy and inexpensive to use. The challenge is with finding the right match for your compressor. You might contact the manufacturer for this, or you can have a professional open the switch cover and locate the pressure switch part number. You can then search online or go the nearest hardware shop and look for a replacement. The cost may run around $20 to $50.