Mice love those enclosed spaces. I learned my lesson the hard way and now I cover the filter chamber on machines stored outside with aluminum cut from frozen food packages. I also remove the seats, because apparently chewed up foam makes a desirable rat's nest.
Years ago, I spent a couple of weeks trying to figure out why the straight six I was rebuilding in my '65 C-10 ran so great for the first few minutes, then died and refused to restart, no matter what I did. Finally, I gave up, and got ready to pull the engine back out of the compartment. When I disconnected the exhaust pipe, it felt heavy, so I took it outside and shook it to see if something was in there. With a sucking sound, a foot and a half of what looked like cooked meatloaf plopped out on the ground.
The whole time I was working on rebuilding, some industrious mouse was transferring the contains of a 50lb bag of dogfood pellets into the awesome steel mouse house that I had provided. The exhaust gas initially passed through the pellets, until the moisture and heat from combustion cooked them into a solid loaf and choked off the engine's breathing.