Ball-bearings

Yes, but what do you mean by cleaning? A lot of the older reels are easy to strip down and service, ie the Penn greenies. I'm assuming you are hearing sand grind. If so, that's not good. You want to have a good workspace and a block of time when you are servicing a reel. Don't do it in a hurry. Take things apart and put them in the workspace in the order they came off. Clean everything, and re-lube. If the ball bearings don't have sand, maybe you don't want to take it down to that level. Often just taking off the spool and attached parts, and cleaning well and putting back together would be where most of the sand will be.

One other thing, try to have a diagram in front of you as you do it. It makes everything a lot easier.