Have you moved to an area with higher humidity. Are you using cookware without a cover? When you heat items using either microwave or convection, if no top the moisture in the food will enter the cookbox and coat the walls. Higher the relative humidity, the more 'sweat' you will see on the walls of the cookbox. I often wipe the walls after cooking to remove any excess moisture.

if it is sweating while your not using it and is vented to the outside ......check the outside vent and make sure the damper is closing....Note: some units the damper is mounted on the rear of the microwave itself.....