Ringing Noise in Gear

Hoping for some insight from the community on a ringing noise I have been hearing. I have a 1995 Berlinetta with a manual transmission. I’m loving my first year of ownership and the car is driving great. That being said, after 30-40 minutes of driving I start to hear a relatively quiet ringing noise while in gear. Ringing noise is present in any gear and across the rev range, without any change in tone under power or off power. If I shift into neutral or depress the clutch pedal the noise will stop within 1-2 seconds. Sometimes it will spontaneously resolve, but only ever presents itself after 30-40 minutes of driving. No relationship to the AC running or being off. Present in either suspension setting. Thanks in advance for any feedback.