They take out the throat plate and bobbin case to clean down there on the 4.2 and 5.2. You might reach something with tweezers and a vaccuume cleaner. Some have even bought mini nozzles meant for computers for vaccuuming jobs. You are meant to hand it in once a year or so for service and cleaning, but it looks like most people leave it much longer, years and years, as long as it runs.
English video for light cleaning on a 5.2;
A Finish video, I don't understand a single word, but the visual illustration is good. I guess that's the part you have been doing. Other than vaccuuming I don't think you are meant to do anything. If you try to remove the covers; side or top of the freearm you are on your own when it comes to guarantees and repairs. I think there's up to 5 years guarantee on these, at least two.