I did have a flexible metal foil vent duct, but it was stretched tight to maintain the 4" diameter over the entire length, so I didn't think this was an issue. Even so, I changed it to a rigid/flexible duct this evening, and the noise is still there. The entire duct run is less than 15 feet to the outside and I cleaned out the entire length just before installing the new dryer. The exhaust duct is rigid / NON flexible metal duct except for the 20" piece between the dryer and duct at the floor. I also checked the exhaust outside the house before and after the change in the short flex duct, plus how the dryer exhausted into the room with no duct at all. Differences in air flow across all three of these cases was negligible. Additionally, the dryer manual claims it will show a fault if the ducting is not adequate. The noise sounds like a slinky hitting sheet metal, like the heating elements vibrating against the heater box might sound. And as I said before, it gets louder and softer, occuring more often and getting louder as the dryer gets hot. Any other thoughts/comments?