hotskillet,

What model car are you working on?

The W123 calipers have locking tabs sticking out from a washer plate that fits under the fastener holding the caliper. Later cars use self-locking fasteners and you either replace them or the self locking goop on the fastener. The new fastener comes with a blob of blue stuff on a small area of the threads which is in all probability a version of Locktite thread locking compound. This stuff is not all gone when the fastener comes out, but it is kind of smooshed into the adjacent threads so it may not be as effective on re-use. If you have a W124 or later car, and reused the fastener without refreshing the locking feature, you may have a bolt backing out. If it is a W123 and you did not use the locking tabs on the washers, you may also have a bolt backing out. Either case is not good.

If you tell us which car you have we can try to narrow down the possible solutions. But diagnosing a noise via text interfacing is particularly challenging. Good luck, Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)