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The whining noise indicates a worn front transmission pump, which will cause a reduction in fluid pressure and result in slippage. On a rainy day when the surface is wet with less traction, it takes less force for the transmission to oeprate, thus you may not completely feel the slippage. The first step is to check the transmission fluid, which is done from underneath the vehicle on the transmission. low fluid can lead to this, however, in 75% of the cases, these symptoms lead to a complete overhaul since loss of fluid pressure from a worn pump, damages the internal clutches.
Rip
Lead Technician
BMW Factory Training, ASE Master Techinician, Advanced engine performance.