The spring likely has a little bit of corrosion. I would wipe and clean the spring as best as you can with a rag. Sometimes dirt can build up between between the spring an preload collar also. But you problly won't be able to get that fixed without taking the shock all the way off and removing the spring. If you can reach in to get any exposed chrome on the shaft an clean it with fork oil that may help also. I doubt the shock needs a service but that could be something to think about. If the little things did'nt work I would just put up with it until the off season and take care of it then, unless you think the bike handles wierd.