Now that both of the plastic covers are off you can see just how much accumulated dust this thing has sucked in unfiltered. *once again make sure this is unplugged!*

The lower cooling system is going to be packed with stuff drawn into the cooling fins. Carefully wipe downwards with a wire brush- the fins are very fragile and any sideways movement could damage the fins. The fins are difficult to access around the condenser pump and hoses.