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rocks or debris in your heat shield maybe. But i'm super confused because you are saying front right in post title, then description front left. However at what speeds does this happen.
As a more unusual possibility, maybe one side of your brake pad is worn out. I had that occur after a service at the dealer, which included a brake pad wear inspection. Took the car home and began to get that metal on metal sound on occasion. Turned out that the back part of the pad was completely gone while the front part (which is what they inspected) looked fine. Oops.
Weird...I just noticed the same kind of noise on my w211 right front yesterday. I'm going to inspect today and replace pads if necessary. Rotor seems fine though, at least on the outside edge.
As a more unusual possibility, maybe one side of your brake pad is worn out. I had that occur after a service at the dealer, which included a brake pad wear inspection. Took the car home and began to get that metal on metal sound on occasion. Turned out that the back part of the pad was completely gone while the front part (which is what they inspected) looked fine. Oops.
Well, it turns out I had the exact same issue except in my case it ended up being the rear brakes and the outside pads worn down to nothing. The front pads must have been replaced by the original owner because they're nearly indistinguishable from new.
I haven't tackled the passenger's side yet but the driver's side rear outside pad was down to the backing(!) but the inside still had a lot of mileage on it. Go figure. Also, I had zero warning of a problem because the rear axle only has a single sensor, presumably on the passenger's side (because there was none on the driver's rear).
Lesson learned...I bought this car a year or so ago (2015) and it's a 2007 but outside the range of engine problems, thank goodness. It only had 32k miles on it, so I made the assumption, stupidly, that I wouldn't have to worry about brakes for a long while. (In my last car, an 05 CLK, I didn't replace brake pads until nearly 90k miles).
Takeaway: In my 07 w211 there is only ONE rear brake wear sensor so the fact that the warning light isn't on doesn't mean you don't have serious wear.
This was the culprit. Driver's rear outside pad. Inside pad same side had significant friction material remaining, but this one was scary.
Pads on right are from the passenger's side, which is where the wear sensor is located. Both could have used a change, and were, but they don't look like the squealing driver's side pad.
This took a few minutes to figure out. The mounting screw for the rear calipers is inside the rubber tube. It takes a male Torx. T14, maybe? There's another on the underside.
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