When I first got my 201 I literally had to scrape grooves and corners clean with a tooth pick. The grime wasn't very detectable, but when I got to round two with oiling and cleaning I still ended up with quite a bit of grime on the tip of the tooth pick. It's odd how dirt and dust build up, and sometimes I need extra good light, glasses on, a bit of time and patience to sort things out. The 401 should run just as quiet as a 503, and I'm sure you will detect the problem.