Revspan

over the weekend, I had a grinding metalic noise when shifting into reverse. It would not happen all of the time, and when it made the noise, it would also go into gear without a problem. The boat is a 2004 Powerquest 260 with a Mag ho/Bravo x outrdive. The boat had about 175 hours on it, and it has been well cared for. I really am not sure where to start to look for issues. Other than changing drive lube every year, the drive has only been off once, last fall, to check for alignment. The technician told me that it slid right off, and that everything looked good. Any ideas? Thanks!