I'm glad you figured out the source of the noise. You could try putting the regular needle plate back on and setting your stitch length to "0." The regular needle plate may be quieter than the darning plate.
It isn't an absolute requirement that you drop or cover your feed dogs - it just removes some friction to do so.
I have long wanted a vintage darning foot for my favorite 15 and I finally got it a few months ago - ohmigosh, that thing makes a racket like you wouldn't believe!!! It sounds like the machine is beating itself to death. :shock:
Hope you can get a lot of quilting done now. :)