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I'm getting a loud humming/vibrating/knocking noise - like someone knocking on a door, only less loud - when accelerating and when I stop accelerating, the noise stops, but the actual vibration with a slight knocking noise doesn't seem to end until the car comes to a complete stop.

So if I let the car roll without accelerating, I'll still hear some of the humming/vibrating/knocking, but it's a lot less.

Details about when the noise occurs:

  • All types of acceleration

  • Steering wheel doesn't affect it.

  • I can hear the noise, but I cannot feel the vibration in the car. If I turn on the radio on normal sound, the noise is barely noiser than the radio.

  • The hum occurs as long as the wheels are moving, almost as if something is rubbing up against the wheels. I don't see anything doing that though.

  • No check engine light or any other warning light. I don't know how trustworthy these things are anyway.

I just want to narrow what this could be, as I've had a mechanic deceive me a few times about problems and I had to return again and again, only to later learn the guy was being dishonest.

Car is a Kia Rio.

Update

Mechanic diagnosed the wrong issue because the sound didn't go away, but not before charging!

The sound only occurs when the car moves and while acceleration seems to make it hum/knock more, even if I let it roll, it still hums/knocks some.

Update 2

The mechanic claimed it's a transmission issue. The check engine light did come on a few days later during what felt like a gear switch (this is an automatic). I am leaning toward believing this, though I am a little confused as to why the car makes a knocking/hummning sound when the car is rolling without any acceleration? I didn't know a transmission issue would do that.

  • Unfortunately, your description is really vague. A normal engine hums when it's on and when it accelerates. While I'm sure you aren't referring to the engine, my point is to show how unspecific this issue can be right now. Please edit your question with as many details as you can provide, including whether this happens under hard acceleration, mild acceleration, both, during braking or steering, whether you feel and vibration through the steering wheel, etc. – Hari Ganti Mar 28 '17 at 18:05
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    Does the frequency of the noise change with the speed of the car? Speed of the engine? No change? – Spivonious Mar 28 '17 at 18:22
  • @Spivonious If I accelerate hard, the knocking will increase. If I don't accelerate and let the car coast, as the car begins to slow down, the light knocking slows down. – WhyAShortage Mar 28 '17 at 18:23
  • Does it slow down when your car changes to a higher gear? – Spivonious Mar 28 '17 at 19:25
  • @Spivonious No, the humming/knocking speeds up; the knocking is not as often as the car coasts slowly. – WhyAShortage Mar 30 '17 at 20:46
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What you are describing could be any number of things. It sounds like something may be vibrating against the body or frame, which transfers audible sound to the interior of the car. It could be anything...bad motor mount, loose exhaust pipe, rough running engine, transmission mount, etc.

You will need to do some work to see what is going on. Safely jack the car up, or put it on a lift, to inspect the components under the car. It could be as simple as a heat shield touching an exhaust pipe after being bent from road debris.

Don't rule out motor mounts. A weak motor mount can allow the engine, when revved, to twist enough that something touches causing vibrations.

  • I had a noise which was exactly like you describe but a little more tin sounding. In my case my vehicle is a 4WD and the exhaust heat shield and broken loose and was resting on the drive shaft. Acceleration or breaking would clunk it, whilst normal movement with or without engine would create small humming noise. In my case I did not event need to jack the car I would just look under it and move it (cold of course) with my hand. – Chris Mar 28 '17 at 19:34

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