One thing I had in my engine compartment that was a great rat deterrent was a brown snake. Not really what you like to find. I didn't keep that one.
I also once had a nice carpet snake that was good at keeping away the rats except that it also had a liking for my chickens. I found it curled up in the rafters chicken shed. It was stuck because it had gone in through the netting but when it swallowed a large pullet it couldn't get out through the netting. I dispatched it with a 12-gauge shot gun and, in the proceeds, blew a hole in the roof. I first found it on my office desk one day, caught it in a prawn net and released it outside in the garden.
