This is amusing, though it relates to deer not rats;
I like the comment that propane gas exploders and strobe lights can be annoying to humans. There is also a comment that there is no evidence that deer can hear ultrasonic devices attached to vehicles. I was sceptical about these devices on cars, but I fitted a pair to my vehicle and I am beginning to be converted (they are passive devices the flow of air when the car moves is supposed to generate ultrasound).
On several occasions, including this morning, I have noticed deer apparently hearing the approaching car at long range then making off fairly quietly into the woods. Often in the past, without the device, deer have been startled and bolted onto the road. It could be chance, but I am coming round to the idea it works.
With the rats though I am beginning to like the idea less and less. I doubt it will keep them away but it may affect other wildlife like bats and owls - and maybe deer.
Where are all these rats coming from, have you had a good look around the area?