Hello Wilkesy91, Hope you are safe and good.
Seeing to your issue, first of all check in the task manager which service/task is taking up space or is running in background. They can be apps, software's, Microsoft Services, Windows Defender running/scanning and its service Antimalware service Executable.
If that is running, then thats means that either Defender is running a background scan or their is virus/malware tampering with your device. In that case I would suggest you to firstly uninstall all the antivirus even the Avast Premium and install Malwarebyte and Run a full system scan.
Along with that, you can also check Windows Update manually as sometimes the ML doesn't pushes you update on time. If other task running in task...