Repairman was just here to repair my 15 year old dryer.
While replacing belt and a couple of rollers, he check the pipe below the lint filter. You have to take the front of the dryer off to access it.
Bear in mind that when I dry clothes I will check/clean the filter at least a couple of times.
He pulled out enough lint to almost fill a plastic grocery bag. Of course, it is a major fire hazard.
I had no idea that the lint would collect there since I kept the filter clean.
He said the main culprit is fabric softener sheets like Bounce. Something on them causes the lint filter screen to become slick and let lint through.
Recommended cleaning the lint filter a couple times a year in warm water and Dawn dishing washing detergent.