As mentioned, the ARV is on '85 W123's, Federal flavor. My white wagon developed a very pronounced whistling sound upon accelerating along with only about 5-6 pounds of boost. I was told to plug the ARV and yes, that fixed my issue. Below I had it plugged with a golf T for testing, and when I determined that that was my problem I inserted a BB in there and plugged the line back into the ARV so it looks "clean".

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Jimmy L.
'05 Acura TL 6MT
2001 ML430 My Spare

Gone:
'95 E300 188K "Batmobile" Texas Unfriendly Black
'85 300TD 235K "The Wagon" Texas Friendly White
'80 240D 154K "China" Scar engine installed
'81 300TD 240K "Smash"
'80 240D 230K "The Squash"
'81 240D 293K"Scar" Rear ended harder than Elton John