It sounds like there is some dried oil somewhere. I thought there was a picture of where on the needle bar that has a tendency to have the need to be cleaned better. I couldn't find it. I did find where Miriam wrote about it at
Singer Slant O Matic 503 , any info helpful
When I am working on slant shank machines, there are a couple of links that I refer to. They are
She's real fun, my 401... and
https://archaicarcane.com/stuck-in-t...lant-o-matics/
Janey - Neat people never make the exciting discoveries I do.