I had plenty of issues with that creaking... Sometimes it was saddle, and sometimes BB.

If it's a saddle - lube the rails where they go inside plastic base of the saddle
With BB, wet and sand will do it... Take BB out, remove all the dirt/sand. Put BB back, and use some good, waterproof grease. I use cheap marine grease. Make sure all the bolts are tight...and you should be ready to go.