* Worn Shock Absorbers (Dampers): This is a very common cause. Worn shocks can lead to a knocking sound, especially over bumps or uneven surfaces. Check for leaks around the shock absorber shafts. They might also be excessively compressed or extended.
* Worn Bushings: Various bushings throughout the rear suspension can wear out. These include:
* Trailing arm bushings: These connect the trailing arms to the chassis. Worn bushings will allow excessive movement, leading to knocking.
* Anti-roll bar bushings (sway bar bushings): These connect the anti-roll bar to the chassis and suspension. Worn bushings cause clunking or knocking, especially during cornering.
* Spring seat bushings: These sit between the spring and the chassis/suspension arm. Wear here can create a knocking noise.
* Loose or Damaged Suspension Components: Check all bolts and fasteners in the rear suspension for tightness. Any loose bolts can cause knocking. Inspect suspension components for any signs of damage or cracks.
* Worn or Broken Springs: A broken spring will be immediately obvious. A weakened spring might not be as readily apparent but could still contribute to a knocking sound, particularly if it's unevenly worn.
* Wheel Bearings: While more likely to cause a humming or rumbling noise, severely worn wheel bearings can also produce a knocking sound, particularly when turning. Jack up the car, grab the wheel at the top and bottom, and try to wiggle it. Excessive play indicates a problem.
* Brake Components: Although less likely, a problem with the rear brake calipers or other brake components *could* potentially contribute to a knocking noise.
How to Diagnose:
1. Visual Inspection: Start by visually inspecting the rear suspension. Look for any obvious damage, leaks, or loose components.
2. Listen Carefully: Try to pinpoint the location of the knocking sound. Is it more pronounced on one side? Does it happen only when going over bumps, turning, or braking? This information will help narrow down the possibilities.
3. Test Drive: Take a test drive over various road surfaces, paying close attention to when the knocking occurs.
4. Jack Up the Car (Safely!): Securely jack up the car and support it on jack stands. Then, carefully inspect the suspension components more closely. Try to move components by hand to check for excessive play in bushings or other parts. This requires caution and ideally some mechanical knowledge.
What to Do Next:
If you are not comfortable performing these checks yourself, take your Fiesta to a qualified mechanic. They will have the tools and expertise to accurately diagnose the problem and recommend the appropriate repairs. Describing the noise as precisely as possible (e.g., "loud knocking over bumps on the driver's side") will help them.
