In April this year, I purchased a 2005 Mazda MX-5 1,8 with 52 000 km on the odo. The car is immaculate and runs well. It’s fitted with the six-speed transmission and has a Torsen limited-slip differential.
I recently performed a major service (all fluids, plugs and filters), but could not fix a metallic rattling sound that emanates from somewhere in the drivetrain, gearbox or differential (and it’s been evident since I got the vehicle). It sounds like a marble being shaken in a tin and occurs only when the car is in gear and moves, and is evident only when the engine is under any kind of load and being lugged.
Putting the car into too high a gear at low speed and revs and then applying a slightly bigger-than-usual throttle opening is when it is most obvious. But it is also noticeable when driving on the freeway. Changing down a gear and running at higher revs seem to hide the sound. The engine never loses power, so I don’t think it is something such as self-detonation or pinging. I have checked the heat shields and under trays to make sure they are all secure and none have any stones trapped behind them. It sounds to me like it may be the input shaft bearing on the gearbox, but I am not sure. It is difficult to pin-point exactly where it originates. Any advice or recommendations would be much appreciated.
JONATHAN GERHARDI
Cape Town
Answer: It is very difficult to find the root cause of a noise as described if the exact location is unknown (and without hearing it ourselves). Therefore, it’s wise to follow a pragmatic
approach to try and solve the problem. Firstly, determine if the metallic sound is engine-speed related, vehicle-speed related, or just random.
Engine-speed related
If the frequency of the noise increases with a rise in engine speed, then the following may be to blame:
- A bearing that is engine-speed linked is about to fail (example: the idler on the auxiliary belt);
- Valvetrain noise;
- Pinging (knock) when accelerating under load owing to poor fuel octane number, tooadvanced timing, hot spots in the combustion chamber, etc.
Vehicle-speed related
If the frequency of the noise is linked to vehicle speed, irrespective of engine speed of gear selected, it’s best to work backwards from the wheels:
- Wheel bearings;
- Brake problem, for example: metal backing plate and disc touching or stone caught between the moving parts;
- Propshaft bearing or universal joint worn;
- Gearbox output shaft bearing problem;
- Differential issue.
Random noise
For obvious reasons, this is the most difficult type of noise problem to solve. Normally, a vibration at the natural frequency of the problematic component will excite it and the noise will become audible. This frequency can be as a result of engine/drivetrain vibration or road surface inputs on the suspension.
The following is possible:
- The heat shields surrounding the exhaust or engine are loose;
- The exhaust hits the underbody;
- There’s a catalyst or baffle loose inside the exhaust.
You’ve eliminated most of the possible causes with your analysis, but there still a few more to investigate. Perhaps others readers have suggestions for solutions?