Your best bet is to contact Shimadzu to find out if you can get a copy of the service manual. I don't have "hands on" experience with that system, but in general if you're lucky it will be something like the cam follower, and a dab of silicone grease on the cam will take care of the problem. If that doesn't quiet things down, then the next likely suspect is one of the bearings in the drive. If it's an open bearing, a shot of spray lubricant might help. In either case, you'll have to open up the case to get at the works. Worst-case is the pump would have to be rebuilt (replacement would probably be cheaper).