If its mechanical failure, I'm not sure there is a way you can test it. Programs will tell you that the drive is working correctly as far as checking the disk surface and read/writes. If it is getting ready to fail mechanically, it will start with making noise. It is probably the component that controls the head movement on one of the drives or possibly a bearing for the disk itself. It will continue to work until whatever it is completely fails. That could be today or 12 months from now depending on use and other considerations. I'd go with the other sugestion and unplug one and see which one it is that is making the noise. If you have critical data, I'd get another drive and back it up to that. If its just programs you use that can be installed and you don't want to spend money, wait till it quits and then replace it.
BEarp