I had that problem with my 6500 C3. The casting button was also sticking, and I took the reel apart but couldn't get it back together again! I got a very good lesson from a guy from another forum on how to take apart the reel and clean it out (cheers to "abbyfireguy" from BC Fishing Reports). What a skill to have developed! If you haven't taken apart a baitcaster before - beware! Anyway, you can likely download a nice schematic of your reel from Mike's Reel Repair website. Anyway, my 6500 C3 works like it's brand new! I also just restored (cleaned) a vintage Abu 6500 C (early 1970's) that was given to me be my great uncle.
One thing to note - don't let water from the Fraser River into your reel (e.g. don't let your reel go for a swim). The water will evaporate, leaving the silt behind that will damage the reel's parts, in addition to being "noisy".