2002 TL-Type S Noise Diagnosis

I am getting a faint noise above 60mph, typically when going at a steady speed. If it were happening during acceleration, I would say the engine is
pinging, but it sounds like is it coming from the rear of the car. It is faint enough that I need to turn off the stereo to hear it clearly.
This car has had all the services (3k miles each), I only use 93 octane gas, etc, so it has been well maintained.
This noise is driving me nuts! Any suggestions? Please?
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