Sounds like the tensioner is bleeding off the oil pressure over night. (I know, thanks for stating the obvious.) Is there a check valve to hold the oil in it? I am not so familiar with the 400E, although it may be like a tensioner for the other MB V-8's. Try a search on this forum for tensioner and rattle. I wish you good luck!
1989 560SEL (172k)
1989 325IC (122K)
2004 Suzuki Volusia LE (3500 miles)
2005 Yamaha Road Star (20 K miles)