At my ranch we have pack rats that try to nest under the hoods of vehicles in the yard. We have found that they like the tight space, and warmth of the engine. The only solution that we have found is to open the hood and leave it up when we are not using the vehicle. It works most of the time, but now the cars in the drive look like there is always something wrong with them. At the same time it sure beats digging out twigs and leaves from all over the engine compartment.