SLK Rear Noise
 

Hi All,
I was driving in Baltimore today when a new noise developed in the rear of my SLK32. At 60MPH, the noise almost sounds like helicopter blades. The frequency varies with the speed of the car, and I can hear it going slowly as low as 20MPH. The noise ends when I mildly use the brakes or gas. I notice it very much when I am coasting. I also noticed that when I drive over painted lines, the noise disappears. I cannot tell if it is driver or passenger side. Any suggestions as to what to look for? Thanks!

First thing I would look for is ridging, cupping, bulges or indents in the tires. Either out of balance, or even possibly a broken belt within the tire can cause these sorts of noises. Then I would move on to the brakes looking for calipers that are not fully retracting and barely touching the Rotors.

I looked at the tires, and there's nothing obviously wrong. I have a tire-salesman friend look at it too though. I don't think it's the tires though because that noise would persist when I'm pushing the brakes or gas, because the anomaly in the tire would still impact the road.

I'll also check out the brakes today, but can you help me understand why the noise would end in the brakes if I accelerate slightly? Thanks.

Oh yeah, and the tire's are new. Only 3000 miles on em.

Any chance this could be related to a flex disc issue or maybe some sort of CV problem? (directed at everyone else not MGPep) I haven't heard the noise yet but by the sound of it he could have a flex disc/plate getting ready to go.

When I was taking the wheel off to inspect the calipers, I found that three of the lug nuts holding on the driver side rear wheel were missing. I suppose they vibrated out. I replaced them and made sure they were ridiculously tight. The sound went away. Thanks for the help though.

Three lug nuts missing? brrrrrrr! You should deem yourself lucky that the wheel did not come off during the drive.

Nice to hear that the problem is identified and fixed.

#1 on list of tools to buy: torque wrench!

worst part is we used mine last time we had the wheel off... 80lbs iirc but we didn't retorque them which was probably the culprit.

Yeah, I was very surprised that thats what the problem was. It would have been very expensive to fix an otherwise cheap problem.

I went on a road trip right after I tightened the bolts. I checked them after the trip (700ish miles), and they were looser again. Would over-tightening have loosened the threads or something so that the bolts are more easily vibrating loose? I can settle with tightening the bolts every 500 mile if I have to though...

I'd worry about cracked wheel bolts. Once you get movement like that, it's easy to fatigue the bolts. Below is an example of a wheel hub unit where the lugs were loose.

Even if I've just replaced three of the lug nuts?

On Mercedes (and many other cars) the lug bolt is a separate piece that threads into the hub through the wheel, not like in the picture you posted, even if the original bolts were somehow fatigued the three replacements would not have been which makes that an unlikely problem. at mrgpep I would just check them once a week for now to make sure this is just an isolated incident, it is not uncommon to have to re tighten after that many miles but if you keep finding them loose every week or two then the hub may have stretched or stripped threads (which is pretty unlikely since the bolts themselves are designed to be weaker than the hub)

The remaining two took up the slack in load. Plus there was movement. Every text book mentions that fastener fatigue can only happen when there is movement within the clamped assembly. With fatigue you get no deformation of the part, so there is no gross warning of the failure. Only a close inspection can tell you if the remaining wheel bolts have fatigue cracks.