Brake pad sensor. I once installed it wrong way and it use to come in contact with something at every turn making grinding noise.
Could not figure it out till i took for other repair at indy and asked him to check it out. He put it right and all is well since.
Last sentence is wrong as car is on jack waiting for new brake fluid as i mistakenly removed caliper half bolt and lost some fluid in rear and have mushy brakes.

Some people(me) never learn to do it right everytime.

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1) '04 S500 Gold/Beige......120k Miles
2) '93 300E 2.8...Green/Tan MBTex..........202k Miles(and going)
3) '89 420SEL...Light Blue/gray Leather....155k Miles (Retired )