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