On Whirlpool Duet and Kenmore Elite HE3 style dryers they will commonly build up with lint inside the blower housing. The blower housing is underneath where the lint filter on the dryer goes. To clean out the blower housing you can either stick brush down where the lint filter goes. Or the better option is to unplug the dryer, remove the lint filter, remove the two screws that hold on the bottom panel of the dryer, and then remove the two-four (depending on the model) screws that hold on the blower housing. Clean all of the lint out real well and then re-install.
When the blower housing is clogged with lint, the dryer will work less efficiently and clothes will take longer to dry. Other things a clogged blower housing can cause is blown thermal fuses, heating elements to burn out, and its also a fire hazard.