Your noise floor does sound a bit high. Make sure your RF gain is as high as possible without getting an ADC overload warning. Also, disconnect your antenna and if your noise floor falls substantially then the noise problem is down to your antenna installation.
Regarding your second question, the scroll wheel on your mouse will only change the frequency if you have selected either the Rx Control Panel the SP1 Panel or the Aux SP Panel. To check this look to see whether the SDRuno text at the top of the panel is red or not. If it is red, then it is selected or ‘in focus’.