Because the fan clutch is engaging and spinning the main fan faster? (I bet it ran near-silent in the winter.) Note the coolant temp when the engine gets loud.

Also, if you recently switched to heavier oil, the engine will groan and vibrate more. Stick with 10W-40 and you will wonder if it is running.

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black