I'm sure you are onto it but; there is a recommedation to use one or two drops of Triflow or slightly heavier oil down the shaft under the worm gear. The same for the worm gear itself, a drop or two of teflon oil can silence the growl on these. You can take the cam stack out when cleaning, it's a bit easier to check the oil points and move parts up and down manually when they are out. Another thing is the area behind the face plate where the needle bar moves up and down, if it's gummed up it can take a bit of effort to flush things out and to get things moving freely again. I like
Archaic Arcanes posts on these, they are very good. There's a few others worth searching up. As I know you know, it's that one spot we have missed or that need a bit of extra attention and it's hard to pin point exactly where. A good way about it is to have the machine out on a table in a heated room, go over all the points the next few days, work a bit on it now and then. The problem spot will show it self sooner rather than later.