Where about on the UIM is the hissing coming from? With the animal issue and the noise, you might have a vacuum leak. Get some carb cleaner and spray it by vacuum lines while the car is running, if the engine rev'*, then you've found the are where the leak is.
i had the same problem thought it was a fuel pump so i put a new one in all that fixed was the gas gauge. it turned out to be a mass air flow sensor so i would have that checked first before you start spending money oh by the way the oil pressure switch on your car needs to be replaced to fix the gauge problem its really cheap to do.
Well, I replaced the fuel pump today and that didn't make a difference, but as soon as I saw that about the MAF I disconnected it and it'* running 'fine'. Not perfectly crisp, but it'* steady and not stalling / sputtering.
That seems to be the problem, I'm going to pull and clean it and if that doesn't work I'll replace it.