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I have a persistent low speed metallic noise in both forward and reverse at very low speeds in my 2009 SL63, call it 5 MPH or less. It oscillates and the frequency increases with speed. The noise was present both before and after replacing pads front and rear, rotors are in excellent condition with no vibration that would indicate warping. Under normal to heavy braking I don't hear it, this is primarily when either coasting to a stop or starting from a dead stop. Light activation of the parking brake does not seem to change the noise. When I jack up the car and rotate the wheels by hand there is no noise, so I'm a little perplexed. I replaced by brake pads using the shortcut of disconnecting the SBC module, as I don't yet have a STAR. My understanding is that there is a calibration in STAR associated with replacing the brake pads and the system adjusting offsets to account for the thicker new pads, is it possible that my SBC is causing the brakes to stay slightly activated at low speeds? Or is this a very slightly warped rotor causing the noise?
I didn't have rubbing symptoms so that could be something else. Pretty sure your mounts are shot if original. Mine were at 26k miles. Replaced 2 engine and the transmission mounts. All smooth now no vibrations or noises.
You can do a simple test if you have a friend to help. Open the hood, friend starts car. Stand in front but beside the car, you don't want to get run over. Friend With foot on brake, puts car in D and guns the engine 3k RPm of so. Gunning the engine causes stress to the mount and if engine moves or rotates significantly, the are shot. I think you can also test in R to test both mounts (not sure, going from memory). Be Sure To Keep Foot on Brake! These are from my SL550 at 26k miles, next to the new replacement Lemforder.