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2011 Mazda6 FWD 2.5L I4 6-speed manual 85,000 miles

5-6k miles ago (2-3 months) I started to notice a "rustling" sound coming from the front of my drivetrain, but only when in gear above 4,000 RPMs. The sound comes from directly beneath the center console, if not a little bit forward and under the HVAC controls.

The sound is generally light--there is no knocking or pinging. It sounds like paper is being aggressively shuffled, or like shaking a branch with dead leaves.

Here are some observations on this noise:

  • Has never occured below 4,000 RPMs
  • Occurs all through powerband above 4,000 RPMs
  • Occurs in any gear
  • Occurs only when transmission is in gear and car is at speed
  • When car is at speed, I press in the clutch, press the accelerator to >4,000 RPMs, no noise is made
  • When car is at speed, I shift to neutral, press the accelerator to >4,000 RPMs, no noise is made

Any thoughts what could be causing this unusual noise, that occurs only when the engine/transmission is bearing the load of the automobile?

My daily commute is mostly highway, and includes many hills between Vermont and New Hampshire--hence why I encounter >4,000 RPM frequently enough to notice.

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  • This surprises me that you run your car routinely at 4k+ rpm ... most vehicles are designed to run in the 2k range when at highway speeds.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Dec 28, 2015 at 17:14
  • Not so much driving routinely at 4k+ RPM - I don't try to push the limits of the car - but 70-75MPH cruises at 3,500RPM in 6th; short on-ramps; 4th gear to maintain 55-60MPH speed limit on hills. I've learned the engine to be pretty anemic, and getting up to highway speed or maintaining speed limits on rural hills means breaking the 4k RPM threshold multiple times a week.
    – Trevor D
    Commented Dec 28, 2015 at 17:35

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Unfortunately I can not pinpoint your issue, but will give you the information I have. Since the noise is not speed dependent, yet does not exhibit itself when the transmission is not engaged (either through disengaged clutch or left in neutral), it is my approximation the noise emanates from the transmission. The noise sounds only when the transmission is under load. This would lead me to believe there may be a bearing going bad (or just being mad) under the conditions you spell out. The bearing would most likely be in the front (engine, not final drive) side of the transmission.

With that in mind, I would suggest you don't do anything with it. Either the noise will become a real issue at which point it will need to be fixed, or it will never be anything but an annoyance. Until some kind of real issue (other than a weird noise) starts exhibiting itself, you cannot really tell a transmission shop what's going on, nor expect them to fix it. The issue may never become a real issue ... it might just remain a noise. If the noise becomes louder, or the noise becomes associated with a vibration or some other form of issue (shifting, for instance), then take it to the mechanic.

One thing you may be able to do is to either have the transmission fluid checked to make sure it's up to level. If that's good, you could possibly have the transmission fluid changed, but I doubt that will do anything. It may be worth a try, though.

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  • Transmission fluid flush did not change this.
    – Trevor D
    Commented Feb 25, 2016 at 15:57

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