My '91 560 SEL does this. I've been told the ticking is actually the rocker arms/cam. It also could be the fact that the previous owner did not change the oil on a regular basis and the lifters are "dirty". Might be a different case with the 6-cyl...

Noise could also be a bad exhaust manifold, something to check.

Also, one cause of lifter noise is bad o-rings on the center bolt on the oil filter cap. They don't come along with the new filter --oil leaks down from the lifter gallery overnight and takes sometime to pump the lifter back up.