RA
the planes that I've flown had old style radar altitmeters...you know the type, an analog needle that moves....and it made a funny sort of scratchy sound when it moved rapidly, like during a press to test.
While flying along in the higher flight levels, the RA would occasionaly swing wildly from 2500plus to just 1000' or so, and the reason I saw it was the noise it made in the course of its normal operation. ( we flew over another plane it got a bounce ).
Anyway, with the modern RA's, I can imagine there is no noise in its normal movement...without the noise there might not be any alerting of the crew.
There are all sorts of old fashioned noises in planes that caused a pilot to become alert. Indeed, our open cockpit biplane forefathers said the wind whistling through the wires would warn of a stall by playing, "nearer my God to thee".
Just something to think about, loud clicks, buzzes, the whole myriad of sounds...even that lovely 400hz sound of the electrics... a traine ear can hear problems of all kinds...UNLESS they are designed OUT of the system.