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Discussion Starter #1
I have noticed just recently that my R172 55 makes a god awful drone noise between 45 and 55 mph (~1300rpm approx.) when in ECO4 mode, if I switch off ECO on the dash and stay in C mode then the drone disappears. If I switch to S or M, I don't get the drone, so I assume it is to do with the rpm and the 4 cylinders being deactivated.

It's loud and annoying... anyone else suffer with this?

Please don't offer advice such as: drive at a different speed or out of ECO Mode as there are lot of contraflows and average speed cameras that dictate 50mph on my normal routes (M1 J28<>42 and cat and fiddle), so I may as well try to get a few more mpg while my foot is forced as it were.... just want to know whether a trip to the dealer is in order.

Not sure whether the R171 or R170 had eco mode that affected lit up cylinders, hence why it is in the section just in case they did.
 

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Have not experienced this on mine. Almost always drive with ECO engaged in C mode, frequently in that speed range. There is no discernible difference in sound between ECO4/8 operation. Is this behavior recent, or has it exhibited this from the beginning?
 

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Discussion Starter #3
I had not noticed it until a week or two ago and have done these routes before. It's so loud that the first time I heard it, I thought it was because my fuel was low and the tank was resonating, but I've tried with full, 3/4 1/2 and 1/4 and get the same drone. I have the music up quite loud a lot but don't think that was disguising it.

I am still waiting to get my brake pads changed, so will mention it to the dealer when it goes in for those, but I guess it will be a NFF.
 

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Discussion Starter #5
It sounds like a drone through a subwoofer in the boot at about 100hz and loud

The tone does not vary just gets louder at 50mph starts at about 40mph (1100rpm) and fades to incoherence at about 55mph (1400rpm).

eco8 it goes away as I said, only in eco4 it is there.
 

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I really don't like the sound of mine when it goes into 4 cylinder either - there is an obvious hollow droning note to it. Sounds like an aftermarket sports exhaust on a normal 4-cylinder car, which makes sense.

It's a lot more noticeable with the lid up for me.
 

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MBUSA's Will Vetter posted a good description of the SLK55's exhaust flap operation on the AMG PL site: http://www.mercedes-amg.com/privatelounge/forums/showpost.php?p=981551&postcount=3

At a steady 50mph under 2000 RPM, the flaps would be closed. It shouldn't produce the loud 100 Hz tone you describe.

Is the tone constant throughout the 45-55mph speed range?

The only other thing I can think of that is engaged in ECO4 near where you say the sound originates is the centrifugal pendulum in the transmission, but I imagine if something was wrong with that you'd have a bad vibration problem, too.

Will be very interested to see what you discover during your visit to the dealer.
 

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I'm very grateful the flaps are closed - the 4-cylinder exhaust note is not one to to boast about or enjoy as a driver.

we're not the only ones to notice this:

http://www.caranddriver.com/reviews/2012-mercedes-benz-slk55-amg-first-drive-review

The cylinder-deactivation system, a first for AMG, can disable cylinders two, three, five, and eight by cutting spark and fuel and parking those cylinders’ valves. The transition is noticeable only in the form of a slight change in exhaust note—it shifts from a rumbling burble to a boomier, less-complex tune—and an indicator light on the dash.
boom=drone
 

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Agree that the exhaust note is different ("a slight change in exhaust note") and not as aggressive, but it isn't a 100 Hz tone that overpowers loud music. And it appeared recently, so something has changed.

I think the exhaust is pretty quiet while in ECO4...just the way I want it to be when driving through my town early in the morning...past the 2nd-shift state trooper's house...where he's sleeping.
 

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Discussion Starter #10
It's definitely become more noticeable in the past 2 weeks and more so with the roof up, which is also a more frequent occurrence over this period.

The tone is constant, but the volume increases from 45 to 50mph and decreases when hitting 55mph.

The exhaust note has always been slightly different between eco4 and eco8 modes, but at 70mph I don't notice the difference at all, nor at 30mph when it switches down to 4 cylinders.

It's almost a labouring type of sound, but not quite. It's quite offensive to my ears at it's loudest, so much so I have been popping it out of ECO mode when it occurs to afford some peace.

There is no 'vibration' other than this 100hz (approx.) boom/drone/bass note.
 

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I've seen some reports of flap mechanism failures causing exhaust blockage and power loss. Maybe a piece had come loose and is acting like a reed in a musical wind instrument. Could explain the single-note tone and volume increase with speed.
 

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I've just taken the car out for a good run and had plenty of time to play with it in eco mode and in 4 cylinder mode there is an obvious underlying boomy tone that increases in volume with engine load.

Using http://onlinetonegenerator.com I'd say it's lower than 100hz though - it's the boomy monotone nature of it that grates vs the much richer v8 note.
 

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Discussion Starter #13
I've just taken the car out for a good run and had plenty of time to play with it in eco mode and in 4 cylinder mode there is an obvious underlying boomy tone that increases in volume with engine load.

Using http://onlinetonegenerator.com I'd say it's lower than 100hz though - it's the boomy monotone nature of it that grates vs the much richer v8 note.
I think you may be right, more like 77hz!
 
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