I did. And this what I found! Going to vacuum this out with a large shop vac and extension hose.
I understand now. I would consider adding a filter housing at the intake for the furnace.No kennel, but we did have two dogs and two cats in the house for a while. The filter for this at the furnace, and I would have to change filters periodically, but they did not look like this! I am guessing that once this got a lot of dust, it would settle out rather than make it to the filter. This is about 30 years worth! I am going to create access panels and clean this every few years. Getting a new furnace as well and I think that will help as well. Only one small dog now so the fur is a lot less. Also, I think we will get an air filter for the house to cut down dust going back to the furnace.