A rising hum or vibration usually means worn fan bearings, which is the normal end of a five-to-ten-year service life. Rattling often means a loose mount or debris. Water gurgling can mean condensate trapped in the pipe.
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Radon fans are continuous-duty motors, so they wear predictably. The most common noise complaint is a hum or whine that has grown gradually louder over months, which is bearing wear. The fan is usually still moving air when this starts, so the manometer may look normal, but this is the failure announcing itself. A replacement fan is far cheaper than an emergency call after the system stops. Rattling or buzzing that appeared suddenly is different. It usually traces to a mounting bracket that has loosened, a coupling that has worked free, or debris that has found its way into the housing. This is often a short repair rather than a replacement. A gurgling or trickling sound is condensate. Soil gas is humid, and in cold weather moisture condenses inside the discharge stack and runs back down. A correctly pitched pipe drains that water back to the suction point. A sagging horizontal run or a low spot collects it instead, and the trapped water both makes noise and restricts airflow. That one is a pipe-routing fix. One more consideration specific to older installs in this market: fans mounted in a conditioned or occupied space rather than outdoors or in an attic transmit far more noise into the house. Current practice under ANSI/AARST CCAH-2020-0523 keeps the fan outside the living envelope, and relocating an interior fan during a replacement often solves a noise complaint that no amount of servicing would fix. Noise complaints peak in winter across Greater Columbus and Central Ohio for two compounding reasons: condensate load is highest when the discharge stack runs through cold air, and the house is closed up so any hum carries further. A fan that ran quietly for years and turned noisy in January is usually reporting bearing wear rather than reacting to the weather.