Close inspection of the plugs may reveal what's happening.

I'm not one who thinks you are actually detonating. Reading your post carefully says that you can hear the sound with the hood open, meaning you're not driving, you're not under load, and your at idle. If your motor was detonating at idle it'd be toast by now.

So, I'm betting on a lifter or other valve train issue. Might wanna pull the valve cover and see if your cam lobes and rockers are in good shape, as well as your oiler tube.

But I do agree that a stethoscope will tell you more.

__________________
-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition