I am having exactly the same problem with my 1998 E320.
Intermittent, not reproducible (of course!) at the dealership service dept. The engine cranks continously for ca. 10 secs then stops - no start. Also the weird buzzing in the dash afterwrds. But the mobile tech actually did observe it in my home garage (just before I had to tow it to the dealership!). He said it looked like low fuel pressure. They have tried replacing the fuel pujmp actuator - no effect on the problem. It has been sitting there again for the last 2 days - starting just fine. I am baffled, and apparently they are also.
At a loss.

If anyone reads this and has any ideas, pls email me at Lippiello@Targacept.com
Thx