My 300SE start to make a whining noise at 3000rpm, it starts at 3000rpm and disappears at 3300rpm. Both driving and standing still.
The sound is most noticeable inside the car, but appears to come from the engine. If I stand in front of the car with the hood open I can't hear it.
Any suggestions??

Two things common to this.
1. In warmer weather, you will see the fans locking up a little more, this is normal and will cause a noise.
2. The front bearing bracket will fail on these and cause a noise, however, it usually will not go away.

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Benzmac:
Donnie Drummonds
1992 500E (very soon I hope :)
1981 280GE SWB
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM