If you hear it late at night, it's a flying squirrel, raccoon or possum. If you hear it in the early morning and late afternoon and evening, it's probably a normal squirrel (like a gray, red or fox squirrel--not sure what you have in PA).
If you have a snake, you won't hear it, ever. Mice are too small to make noise unless they're nesting directly over your head. If you've not seen rats around your foundation and other places on your property, it's probably not a rat.
Whatever you've got, it's too big for a snap trap and maybe too small or smart for a live trap. It's entirely possible that you have multiple critters and multiple types up there. When I moved into my house I had both gray squirrels and flying squirrels in my attic. The grays slept there at night and the flying squirrels slept there during the day. The mother F'ers were hot-bunking. Plus I had a few field mice thrown in for good measure and at least one rat snake in the crawl space living high on the hog.
Here's my advice:
Set all of your traps and keep them set. Sometimes you're just unlucky.
With your snap traps, screw them to the top of a rafter at the edges of the attic. Then bait them with peanut butter and check them every day. If you have something small like a flying squirrel, it can get out of a mouse trap or drag it away. A rat trap will usually kill a flying squirrel but if you catch a larger squirrel or other animal with a rat trap, it will either wriggle out or drag the trap away. Screwing them down keeps the critter stationary. I've caught flying squirrels in a rat trap and they stayed alive long enough that if the trap were not screwed down, they would have dragged it into an inaccessible place to die--and smell.
If you've got something bigger, you'll get it with a live trap. Flying squirrels and mice will often trip a live trap but just walk right through the gaps in the bars. Don't get discouraged. It sometimes takes several settings to get a bigger critter with a live trap.
I had terrible luck with live traps for larger stuff until I modified the traps. I bent the triggers so that they were hair-triggered. Then (and this is the important part), I used thin sheet metal to wrap the traps around the outside, which denies the critter access from the side, through the bars. I figured out that larger animals will sometimes find the bait and just reach through the sides of the trap to get it (the shortest route) instead of walking around to the end of the trap and entering it. By wrapping the trap, the critter has no choice but to go into the trap like you want him to. This also helps keep your fingers away from his sharp teeth and claws when you carry the trap out. My catch rate went way up once I did this. I secured the sheet metal with rivets.
Even on large traps, I use peanut butter. Critters love it and it's sticky so they have to move the trigger around when they scrape and lick it--increasing your chances of springing the trap and catching them.