motor noise

I am wondering if someone can tell me. when I rotate my props by finger ( machine not powered up of course) What is the noise I hear only on two motors. It sounds like sloppy ball bearings. the others are very quiet and smooth. Thanks Keith Kuhn

Was doing pre-flight this morning and noticed this. The first motor I spin is silent the second one, well you'll hear. Decided to hover at about three feet and listen as well as watch for any abnormalities but she seemed to be ok. This definitely concerns me though. I didn't have her powered up...