Here's why:
* Coasting: Wheel bearings are most stressed under constant rotational speed without the load of acceleration or braking. The knocking is likely the sound of a damaged bearing component making contact and impacting, or a lack of proper lubrication leading to friction and noise.
* Acceleration and Braking: When accelerating or braking, the forces applied to the wheel and hub actually help to temporarily clamp the bearing components together, reducing the play and thus the knocking sound.
However, it's crucial to consider other possibilities, though less likely given your description:
* CV joint: While less likely to present *only* while coasting, a worn CV joint *could* produce a clicking or knocking sound. However, these sounds often change pitch or intensity with turning.
* Suspension components: Loose ball joints, tie rod ends, or control arm bushings *could* contribute, but again, the fact it disappears under acceleration and braking makes this less probable. These would usually be more consistent regardless of driving conditions.
* Brake components: While less likely, a slight drag on a brake caliper could create a noise, but this would probably be more consistent or present at lower speeds.
Recommendation:
You should not continue driving the car with this noise. A failed wheel bearing can lead to a complete wheel seizure, resulting in a loss of control and a dangerous accident. Have a qualified mechanic inspect the front wheel bearings (both sides) as soon as possible. They can identify the exact source of the knocking and perform the necessary repair. Ignoring it could lead to costly damage beyond just the bearing.
