I know the one you mean. I just had a good look at mine and spoke to DH (an engineer). If you had a jewellers loup or other strong magnifying glass you may be able to see it. If not, he suggested a thin strip of really fine emery paper - cover the feed dogs and needle hole so you don't get any dust in the machine.
Run the strip between the loop and the black screw holder a couple of times, then turn the paper to the other side so it's against the black side as you don't know what side the burr is on.
I thought of running a piece of fluffy yarn or embroidery thread to see if you could 'feel' the snag. One last thought - do you have any silicone spray? A tiny drop on the end of a cotton bud may help lubricate it enough so it doesn't catch. Good luck!