You should try to get them as quiet as possible
The humming means that some pushrod is pushing or pulling on the servo arm making the servo motor "fight" to keep centered.

It's very hard to get them all totally quiet, but if you have some spare time, sit down and adjust the linkages so you get as little resistance/bind as possible...If not for anything else, for peace of mind

...I don't know about old servo reliability, but perhaps some new 12$ servos will be cheaper than a crash because the old ones decide to go and die?