I installed a new dryer last year, and within months clothes weren't
drying properly, and then my laundry room started getting steamy...
turned out that the screen on the exterior vent was damaged, and
squirrels were travelling the whole length (appr. 30') of horizontal
venting and nesting right next to the dryer. Lucky for us (and the
squirrels) that we didn't have a fire.

with a chimney brush and a shop vac, we got it pretty clean.