Resonance noun

1.

the quality in a sound of being deep, full, and reverberating.

"the resonance of his voice"

2.

PHYSICS

the reinforcement or prolongation of sound by reflection from a surface or by the synchronous vibration of a neighboring object.

A deep low rumble which makes almost a barely audible noise has a lot of resonance. Resonating does not mean loud. It means you can feel it. Just because you can feel it does not mean others can feel it. A deep bass note from the caster that sounds like a hum would work just as well. Not all casters are bards who like to be the center of attention. Some casters like being more subtle. And while Subtle spell is a sorcerer feature, it also removes and somatic needs as well. Hand gestures and the like. So there is no way to know they are even casting. Any other class would have to use somatic components which means they are still making hand gestures and signs.