There is an instruction step by step on the Dimplex site to open the unit up and clean it. It collects a lot of dust due to the air it moves over time. I use high pressure air to blow it all off and the problem is solved. I have had to do this twice in 7.5 years of full time use (8 winters).
Tip: Take a picture or two of the wiring before you unplug. There aren't many to remove, but you sure want to put them back correct.