Spend enough time on the automotive classifieds and you’ll end up finding a deal that’s too good to pass up. The latest of these in one’s own case was a Mercedes-Benz sedan, just past its twentieth birthday and in surprisingly tidy condition. At less than $3,000, the 1998 E240 was too good to pass up and simply had to be seen.
The car in question. Clean bodywork is too tempting to resist, even if there are mechanical issues.
The car was clean, too clean for asking price. Of course, a test drive revealed the car had one major flaw – an annoying hum from the drivetrain that seemed to vary with speed. Overall though, mechanical problems are often cheaper and easier to fix than bodywork, so a gamble was taken on the German sedan. The first order of business was to diagnose and rectify the issue.
Characterise, Research, Investigate
The first step to hunting down any noise is to characterise it as much as possible. In this case, the noise was most noticeable when the car was traveling at speeds from 40 km/h – 60 km/h, present as a vibrational humming noise. The location of the noise source was unclear. Importantly, the noise varied with the speed of the car, raising in pitch at higher speeds and dropping as speeds decreased. Engine speed had no effect on noise whatsoever, and the noise was present regardless of gear selected in the transmission, including neutral.
A worn wheel bearing can create an annoying noise when travelling at speed.
All the above tells us that the noise is not coming from the engine, and likely isn’t coming from the transmission either, barring perhaps the output bearing. The fact that the noise varies with speed tells us tha ..
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