I was told this by my neighbor a few years ago and since leaving the hood up haven't had a mouse nest in my tractor since. Along side our garage is a carport, I leave the hood up on my truck when parked there for any length of time too. Otherwise they build in the airbox.
A couple months ago I had my car parked under the carport for *one* night w/o the hood propped open. Next morning I saw I had forgot to leave it open, I opened it to find a fist size nest of shredded paper and, what really made me mad part of my ShamWow towel, on a ledge under the master cylinder. A day or so later it was parked there (hood open so no nest). I went out in the morning, opened the trunk and set a piece of cardboard box on the floor. Then went in the garage and removed the battery in my Mule, probably 15 minutes, and set it on the cardboard and drove 25 miles to town. When I opened the trunk there was a black snake laying beside the battery. It must have crawled in while the trunk was open. Anyway I drove back to the garage, took Blacky out and instructed him in my sternest voice that he was to rid the place of vermin! Then I put him in the garage where he slithered under some shelving. Mice had eaten a corner of the rubber sweep off the bottom of the garage door so he could come and go with them. I must say I didn't noticed any new signs of mice until just last week.