Here's why:
* Wheel bearings support the weight of the car and allow the wheels to spin freely. When a bearing is worn or damaged, it creates increased friction. This friction generates heat and a humming or growling noise.
* The noise is more pronounced during left turns. The increased load on the left front wheel during a left turn (due to centrifugal force) puts more stress on the already compromised bearing, making the hum louder. A right turn reduces the load on that specific bearing, making the noise disappear.
* The speed (30 mph) is relevant. The humming is likely only audible at a certain speed range because the frequency of the vibration created by the failing bearing is proportional to the wheel's rotational speed.
Other less likely possibilities include:
* A tire problem: An unevenly worn tire or a tire with a bulge could *potentially* cause a hum, but it would likely be present regardless of the turning direction.
* CV joint issue: A worn Constant Velocity (CV) joint could create a clicking or clunking sound, more so than a consistent hum. While a severe CV joint problem *might* cause a hum, it's less typical and often accompanied by other symptoms.
However, given the specific circumstances you described (left turn only, hum at a specific speed), a faulty left front wheel bearing is the most probable cause. It's crucial to have this checked by a mechanic as soon as possible to prevent further damage and potential safety hazards. A failed wheel bearing can lead to a sudden wheel seizure, resulting in a loss of control.
