I recently bought a 2005 Boxster with 21K miles on it. I noticed that at highway speeds there is a mechanical whine that goes away when I depress the clutch and let the car coast at the same speed. The whine is at a constant pitch even when the speed or RPM changes, just the volume seems to increase when the car warms up more. At low speed I cannot hear it. Any ideas if this could be a bearing starting to fail?

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This noise could be a bearing. The way you describe it going away when you press the clutch leads me to believe it is a clutch bearing. It sounds to me like the throwout bearing is the culprit. The pilot bearing is not spinning when the clutch is engaged, (clutch pedal up) so it is unlikely the pilot bearing is making the noise.

- Nick