Hi guys,

I own a 96 e320 here in hawaii and this noise happens everytime on a cold start.
Has been now for the past 2 months. It's a loud high pitch whirring/squeal (like a mini siren) and after 45 secs it goes away. For example, in our community we have the siren that lets us know if theres some kind of warning (war or disaster), the noise comes on, whines, and then when it shuts down it dies down like the siren does (i hope that was a good analogy haha). It usually comes back again for maybe 5 seconds once the car goes back to idle (like if i start it and take off, at the next traffic light itll come back on for less then 5 secs). The noise will not come back at any other time unless the car sits for hours before the next start.

I know it's not a drive belt. I'd think its some kind of pulley or bearing but the type of sound it is and then cycles off within a minute doesn't seem right. I've given up and now see if someone else has dealt with this issue before. I best guess is some kind of vaccum or ac noise. All components on car works fine and nothing changes the pitch or length of the sound while doing anything (turning ac on, driving, etc)

Thank you for your time.

that would be the air pump. it introduces air in the exhaust during cold start up

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1967 250 S
1967 280SE
1968 250S
1971 280 SE
1973 220
1974 280 S
1975 450 SEL
1976 220 D
1979 300SD
1983 300 SD
1985 230 TE euro
1986 280SL
1986 500 SEL euro
1986 190E 2.3
1990 300 SE
2003 C230 K
2004 C230 K
1987 560SEL
1987 300SDL now in canada
2005 C230
1998 C43 AMG Sold to DON
1993 190E 2.6
300TD Former daily driver Totaled
current:
R320 CDI
Ram 2500 Cummins

good search terms are air injection pump or smog pump.....