Greys and flyers are in my attics together. so are bats. . but noises are at different times and are different in pattern and sound. Flyers at night bumps, fast rapid running in short bursts sounds like marbles being rolled around. Most active on bad weather nights all night other wise noise for 2 or 3 days then qt or very little noise for 2 or 3 days. the alot of noise again. Greys adults right before day break all day off and on then again just at and right after dark. young will be all day with alittle noise at night for short time frames in one spot. Mice scratching noise (chewing) in one spot bang on spot noise stops then starts up again in a short time. alot of my squirrel jobs the wintering bats (one or 2) are making noises right at dark, most of the time on a exterior wall at a corner.
Go up in the attic and have someone tap on the spot below where they hear the noise, go to that spot and pull back the insulation look to see what you have.
Here is one that I did last week where the folks was telling me they have grey squirrels in the attic, after finding no entry points on the outside and them telling the time they was hearing the noise at night this is what I found Mice and nothing but mice LOL