see also:

• car battery running flat or car not starting
• combustion leak / head gasket leaking
• car over-heating or losing coolant
• pot holes and reducing damage to car

• could be issue with any of the moving parts in the engine bay or gear box/transmission
  • if rhythmic, tapping, could be valves, connecting rods or pistons
  • if squealing at start up or when accelerating, could be worn or loose accessory belts that drive things like your power steering pump, air conditioner compressor and alternator.

• comes from all wheels:
  • probably road surface noise
• only comes from one wheel:
  • if very loud, high pitched and started after being on gravel road:
    • probably a stone is lodged in brake pads
  • if low pitched:
    • may be damaged mud flap contacting tire
    • may be due to worn tire especially if uneven wear
    • may be due to wheel alignment issue
    • may be wheel bearings are worn - these should be fixed as they may result the wheel falling off

• most likely an issue with the brake pads

• most likely an issue with tires:
  • stone jammed in tire tread
  • damaged tire
  • stone caught in wheel and moving
  • loose lug nut inside a hub cap
• a cyclic noise may also occur with worn wheel bearings
• a squeak may be worn universal joint (U-joint), which are found in pairs and are components of the driveshaft

• worn constant velocity (CV) joints
• worn bushings (rubber parts which reduce the rubbing of metal parts) - knocking occurs when not accelerating or braking and especially turning corners
• worn ball joints - cause knocking when turning the steering wheel and get worse the more you turn the steering wheel

• this is usually a damaged suspension component
• damaged strut or strut mount
• broken sway bars - this may cause you to lose vehicle control when rounding corners at speed
• loose wheel nuts (lug nuts) - especially noticeable at faster speeds - needs addressing to prevent uneven tire wear, damage to suspension or worse, the wheel falling off!
• shattered wheel bearings may cause knocking noise when driving straight and eventually the wheel may fall off
• worn control arms on front wheels will cause some knocking noise when driving slowly and also a rough driving experience
• bent axle from driving over a pothole or a large rock - knocking sounds when driving over bumps, shaking that gets worse the faster you go, uneven outside tire wear, and turning issues
• exhaust system issues

• bounce front of car up and down and try to ascertain either:
  • excessive movement of strut mount, or,
  • using a screwdriver as a stethoscope, ascertain which part is making the noise
• have assistant turn the steering wheel while you observe under the car
  • check tie rods and steering knuckles as well as suspension components for excessive movement
• have two assistants shake the front roof with force
  • lie under the vehicle with your underhood work light and check all the components mentioned above that should be causing knocking noise. These include the ball joints, control arms.
• check the exhaust systems – the muffler, catalyst converter, tailpipe, and head pipes.
  • check the connections and hanger that holds them in place. 
  • once exhaust is cold, wiggle the pipes and see if it’ll replicate the knocking sounds