If you can possibly find an old Bernina 930 or a Bernina 801 both of these old machines purr like kittens. My 801 is almost 20 years old and you cannot even hear it run when it stitches, AND it does go 90 mph. Just has a few basic stitches, zig zag etc. Only problem no stop up/down on it, but I still take that to workshops as its lighter.
The old 930's were the last all metal machines. Mine still runs like it did the day I bought it. Upgraded to a computerized machine several years ago, but still fall back on the older machines when I am piecing. Good luck on your search for a backup machine.