The Short Summary A healthy AHF typically produces two types of noise: A low hum from the internal reactor (inductor) and cooling fans. This is normal. A high-frequency "switching" whine or buzz from the Insulated-Gate Bipolar Transistors (IGBTs). This is usually very faint. If the noise becomes loud, intrusive, or changes significantly, it is often a sign of a problem. Detailed Breakdow...