How do I differentiate a sound that may be a bad "air pump" from another type of noise at startup?

It does seem to be at startup. It is sort of a "stress" sound. It isn't a whine, not a grinding or clanking. A steady sound and seems to go away after a minute or two. Usually after the car has been sitting. I have been advised it might be an "air pump" but was also told it might be a clogged oil screen. Any good way to make a best first stab at a fix? The air pump supposedly is not urgent but the clogged screen is worrisome to me. Thank you