Your struts are bolted into the trailing arm. Make sure they are torqued correctly. Also make sure the upper bolts are torqued. Otherwise does the noise occure on a particular road, movement etc. Can you consistantly reproduce the noise? The strut does have internal mechanics and they can go bad. So if the mounting bolts and nuts are all tight you could have an internal failure.
Before going down the strut failure road. Check all rear suspension bushings, connectors etc.
Have someone drive the car and lay in the back to see if you can identify which strut it is or what area it is comming from.

Dave

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car