Same here: the RNS510 seems to get too hot, then the fan kicks into overdrive and remains on to cool the unit even if I turn off the ignition and leave the car...
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Dropped into the local dealer today to order the new key.. while there I asked them to check the strange whilne in the car...
Apparently it is the turbo spinning up and it is "normal". That does not explain why it has only just started happening in the last 5 - 10K though...