Hi,
My car has done nearly 110K miles. Lately, I'm hearing a whistling/whining sound from under the back seat from 50mph upwards. Over the months, this sound has increased gradually. Recently, I had the oil in the differential changed, but this has had no positive effect.

I would appreciate to her your ideas about what causes the sound, and what needs to be repaired/replaced.

Thanks!

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2011 Prius