I have enjoyed reading this whole thread as I've been having trouble with my Elna Excellence 720 (made by Janome) - it's been quite noisy when I use the "dual feed" foot to quilt. It sounds OK when I'm just piecing. The other day, the noise was really bad so I did another cleaning (despite having it professionally cleaned a few weeks ago). I dug way down and got out a blob of lint. I must have jostled something because now it's not as noisy. I will take a look at the bobbin case, though, because it jumps around more than I think it should. I use Schmetz needles with Aurifil thread most of the time. Despite the noise, the stitches look fine.
And BTW, I lost one of the screws that holds the throat plate on down in the machine. I heard it drop all the way, and sewing with one screw was quite noisy. I tried to order a replacement screw from that onlinesewingcenter (or whatever) and they sent a different one than advertised which didn't fit. It was cheap so I didn't send it back, but I'm happy to know about ken's sewing center. Will try them next time I need a part.