It likely will pay off to keep up a daily oiling application for a while, test sew, poke around a bit, move levers and knobs. It will eventually get to the source of the problem. When I take out a machine I haven't used in a while, I need to fresh up my memory and check where all oil points are. I have to say, I had my black cast iron 201 out for a spin yesterday and I was reminded I need to get a slightly wider motor belt (it's not the potted version), and I need to tidy up in the drawer with bobbins and spools of thread. I know you are all onto the details of your 401, but at least for the inspiration
this is a good video.