We had a critter in the attic space above our bedroom the other day and I realized I don't know how to access that area of the attic. We have access to an attic area from the garage on the far right of this picture and also above the second story near the middle of the house. The bedroom is the far left and I think there would be access there but I guess not.

How likely is it that there is no access at all here? I would think based on the ceiling in the bedroom that there would be space but maybe I'm overestimating the height above the ceiling.

I thinks it's unlikely there is a no access and isolated from rest of the house.

If it were me, I'd trace the ducts from main area and see how/where they enter the left bedroom. There is likely access there, maybe not ideal though.

Good luck, critters in attic are no fun.

Hopefully what you are hearing is on the roof and mostly when cold weather amplifies sound.
When a tree sheds large acorns or a squirrel/raccoon jump from tree to roof, one believes they are in attic.
In Aggieland, we are offered free roof inspections but anywhere $100 gets one & even minor repairs such as a critter might make.
Even in this small town we have critter trappers for hire that leaves you hands & disposal worry free. Some will even seal up the hard-to-find entries with metal.

The residential building code has strong fire chief inputs and one of them is that firemen must have access to all attic (on fire) areas to put it out. Sometimes your local building inspector will allow a smallish attic access only via an attic crawl space in lieu of a hatch. Large enough attics may be served by limited length catwalks (plywood walking surface top of joists).
ref 2018 IRC: https://codes.iccsafe.org/s/IRC2018P4/chapter-8-roof-ceiling-construction/IRC2018P4-Ch08-SecR807

So if attic spelunking, follow the roof ridge on left when crawling left.
If the framer installed a "wall" of drafts stopping as required by attic SF codes, then "legally" you must self install a new ceiling attic hatch or fabricate a code approved hatch in that "wall". The ceiling solution would serve future trips better. Some have installed a large enough removable gable vent or Lexan skylight to serve as access.

We have definitely had rats in the attic. I've killed a half dozen or so. I have a trap in the adjacent attic and a camera on it.