Sometimes we don't have the privilege of gathering room tone, or room tone changes midway throughout a clip (A/C turns on, for example).
Ideally, you could run a process (similar to Capture Noise Print) that takes your selection and looks for the quietest bits. Using this, it would generate tone that would match the room tone/ambience of the clip.
iZotope RX 6 has a similar feature under the name "Ambience Match," which is an absolute lifesaver.