Lint build-up, squished flex hose
West Palm Beach, FL (2002) - This image was actually taken "as is" behind a dryer in a brand new model apartment. When the home owner moves in, I hope they are inclined to look behind the dryer and adjust its location.
This is an image of a piece of dryer flex hose that was actually in use at a senior living facility. It demonstrates very clearly how the moist lint collects in the rough area of the foil flex. If aluminum flex was used, this would not of been as pronounced. Checking your dryer exhaust at least every 18 months is crucial!