Smart clean is off. Scheduled tasks are all set to no. It's closed and not minimized. Pro version. Yet CCleaner is constantly running in the background using a minimum of 10% of my CPU and sometimes as much as 30%. Average is a 12-13%. I can't even end the process. It's a problem, especially when editing video files. Suggestions?
Smart Cleaning, Update CCleaner Automatically, Performance Optimizer, and Schedule are the things in CCleaner that may run in the background.
(The first 2 also result in Windows putting a CCleaner icon in your System Tray).
Double check your settings for all 4, but from what you describe I'd suspect that what you are seeing running in the background is probably Performance Optimizer.