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I have a 2001 Pajero/Montero III 3.5v6 GDI automatic which developed a tiny hum about 8 months ago. This hum has been growing louder ever since and is at a point where you can very easily hear it inside or outside the car.

  • The sound is always constant at idle or at speed.
  • The sound does not change if I full lock turn the front wheels at stand still.
  • The sound is only affected when the revs dip below 600 rpm.
  • The location of the sound appears to be more pronounced under the front right seat of the car.

What I have eliminated

  1. I've eliminated the fuel pump as I can turn the key without engaging the starter and the fuel pump makes no noise.
  2. I've replaced the power steering fluid which has zero impact on the sound.

UPDATE 19 Jan 2024

Took her into the mechanic for the yearly and checkup. He reckons its the fuel pump. What another 4x4 enthusiast mentioned was that the pump can dislodge from its bracket and start touching the edge of the gas tank causing a sound to resonate through it.

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    What happens to the sound when RPM drops below 600? Change in volume or change in pitch?
    – MTA
    Commented Nov 17, 2023 at 13:22
  • @MTA Change in pitch lower. Commented Nov 20, 2023 at 13:57

2 Answers 2

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Have you considered a minor exhaust leak? I fixed several slight leaks following this easy video. The repairs have lasted several years, and counting!

Exhaust pipes are hottest close to the engine, with connectors commonly under the passenger seat. These parts are often the first to break.

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I don't know if the Montero is one of them, but there are a few older vehicle models whose cabin ventilation system runs at low speed at all times when the engine is running, and it can not be turned off.

It's possible that a bearing in your heat / AC blower is worn out and allowing the blower wheel to resonate with a low, constant tone. When your RPM drops below a certain speed, your battery voltage can drop a little, slowing the blower, and this could change the tone of the noise.

Find the fuse that protects the heat / AC blower, and remove it. If the tone disappears, you've found the problem. If not . . . well at least it didn't cost you anything!

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