Author Topic: Whining Noise >40mph linked to road speed and engine load.  (Read 6314 times)

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Offline RussB

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Whining Noise >40mph linked to road speed and engine load.
« on: December 01, 2016, 01:05:12 pm »
I've had a search but I can't find anything similar although I'm not sure exactly what to search for.

I've just bought a Mazda 6 mk2 2008 5 speed*1.8 petrol with 85k on the clock.

I get a very noticeable whining noise above 40mph when I put my foot on the throttle.

The harder I accelerate, the noisier it gets.

The faster I go, the high pitched it gets.

The noise is the same tone in 4th and 5th gear - so it is not related to engine speed, but it is related to engine load - if im coasting, its almost inaudible, if I floor it, it whines like a good'un.

I'm thinking, output side of the gearbox isn't happy so maybe something inside the box or maybe a dodgy CV? I'm a mechanical engineer but I don't do automotive stuff so I have no experience of what either might sound like as I've never experienced them)

I have checked the gearbox oil and level, it was ok, not too dirty. The fill plug was correctly torqued so I assumed its been serviced by someone competent or possibly not at all although the oil didn't look 84k old and was filled right up to the plug, perhaps an egg cup over.

I'm not sure, I hope its just a CV. Does anyone here have any experience or thoughts/ideas?

Many thanks,

Russ

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« Last Edit: December 01, 2016, 01:07:25 pm by RussB »

Offline 678hug

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Re: Whining Noise >40mph linked to road speed and engine load.
« Reply #1 on: December 01, 2016, 02:44:59 pm »
Possibility of a worn differential output bearing in the gearbox.  Had it on another model of car and quite a few examples on youtube.
You might want to try put it under load and then selecting neutral to see if the noise disappears.

Offline RussB

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Re: Whining Noise >40mph linked to road speed and engine load.
« Reply #2 on: December 02, 2016, 08:33:06 am »
thanks 678hug - it feels kind of daft now, but I'd pretty much forgot the diff was in there with its a been a transverse gearbox! out of sight, out of mind I guess ;D

I bought this car because I wanted something reliable  ::) I had a 2006 facelift Mazda 6 diesel from nearly new, upto a 135k miles and it never missed a beat, just brakes, pads, tyres, the odd wheel bearing - normal consumables.

Offline 678hug

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Re: Whining Noise >40mph linked to road speed and engine load.
« Reply #3 on: December 03, 2016, 02:53:02 pm »
Now i remember what i did to diagnose a differential output bearing on my previous car. Put under load until it makes the noise the press the clutch pedal. This unloads the gearbox and if nothing else, isolates it to there.

Offline RussB

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Re: Whining Noise >40mph linked to road speed and engine load.
« Reply #4 on: December 06, 2016, 10:29:30 am »
I'm not sure I follow sorry  :o

I think I've already isolated it to the gearbox, specifically the output/secondary shaft. I say this because changing gear doesn't change the note or volume of the sound, so changing engine speed doesn't affect it - thus its nothing to do with the engine. This is also changing the speed of the input/primary shaft (which is linked to the engine via the clutch) and also doesn't change its tone/sound either so I wouldn't expect it to be a bearing or clearance on the input shaft in the gearbox.

So that narrows it down to the output shaft of the gearbox and it's bearings and clearances, the differential, and the driveshafts/wheelbearings (a long shot)

I will do some more tests (slightly dangerous ones). If I jack one wheel off the ground, it will rotate at twice the speed through the differential as the drive from the other wheel will be passed through the spider gear, accelerating it. This will allow me to double the speed on the side gears in the diff giving them a run for their money. I will confirm this theory with actual wheel rpm with one and both off the ground (I have an optical tacho) - the shorter drive shaft side should spin a bit quicker naturally, if it doesn't or there is a significant difference that could be a clue, but the sum of the 2 wheels should equal the single wheel speed at the same rpm (I think....)

It will be difficult to compare the noise of this test to the normal driving noise since I won't have road noise, or proper engine load (I can and will log engine load as I have a gadget for that, and I can then compare that to the engine load I get at say 50mph cruising just for reference). This stationary test might just tell me if I can isolate the noise to the left or right half, at which point, I'll take a gamble and fit a re-manufactured drive shaft and change the wheel bearing while I'm there (not cheap jobs, but I can do it myself). If the problem persists (and it probably will) then I'll have to get a gearbox for it (not expensive, but costly labour having it installed). Ideally a 6 speed from the 2.0MZR if they're mechanically compatible with the existing gearbox/(new) clutch, driveshafts and control cables etc, as the gear ratios better suit my 100% stop start round town commute.

Offline RussB

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Re: Whining Noise >40mph linked to road speed and engine load.
« Reply #5 on: February 04, 2017, 09:51:13 am »
2 month update - not much to say, and they say no news is good news.

The old girl still whistles under load above 40-45 ish, but it isn't any louder than it was 2 months ago and I've probably covered nearly 800-1000 miles at a guess.

It seems to be doing 31-36 mpg depending on how and where I drive, I mostly just crawl to work in traffic through 3 different villages. 31 was surprisingly good as I was booting it about most of the time. 36 was a pleasant surprise, I didn't really do much motorway mileage so I assume if I do a long run it might do a 1 or 2 better than 36 which is great for such a big car (my 2.0 diesel DPF 6 only did about 42 and at the time diesel was much more expensive, it was hardly worth it, and certainly not worth the more complex maintenance and expensive oil etc)

Happy motoring so far, it's exactly what I expected and I should have never bought a bloody German car - never has so much bulls**t being spewed about a countries engineering ability - give me a Japanese or Korean car any day over that tripe!