What Causes Engine Noise When AC Is On but Not When AC Is Off?

[VonEmily]

Determine whether the noise is coming from inside or outside the vehicle. If it's inside, the issue is likely with the blower motor. If it's outside, the noise usually comes from the compressor or other pulleys. Below are the details: 1. Interior Noise: A damaged blower motor, water ingress or lack of lubrication in the shaft, or even shaft wear can cause a squeaking sound. Though not very loud, the noise is somewhat sharp and easily noticeable. In such cases, the blower motor usually needs replacement as repairs are often not feasible. 2. Exterior Noise: Noise outside the vehicle when the AC is on isn’t necessarily from the compressor, though that’s the most likely source—it could also be from other pulleys. Before turning on the AC, the overall load is minimal, allowing the pulleys to operate normally. When the AC is activated, the increased load may cause already worn-out pulleys to produce noise. If the noise is minor, you can continue driving, but if it’s loud, it’s best to have it checked at a repair shop.