The sound is similar to the serpentine belt tensioner/shock that was failing on my 1991 350SD. Changed it out and the noise went away. The dealer mechanic told me about it when he walked by the car with it idling - kind of recognized the sound without lifting the hood.
Seems you have plenty of steps to go through to isolate the noise already. If more people listen to your noises, you will get more suggestions and you won't have any free time for the 4th of July celebration. Good luck, Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)