I have had dirty and worn brakes causing a chirping sound on the 280SEL before. I'm not sure what exact sound you are hearing, but I thought I would throw in my $0.02.

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Ben

The MBs:
1976 300D (W115) - 330K and still going (sort of)
1991 300D 2.5 Turbo - Sold at 221K
1983 280SEL - Sold at 206K
1981 300SD - Sold at 232K