1990 LeSabre

I have a 1990 LeSabre with 215K miles. The last two times I have changed the oil the first time I crank the car up to drive it off of the ramps the engine knocks like there is no oil but as soon as the oil pressure builds up there is no knocking. It only knocks for a second or two. I have had the car for five years and have put 115K on it myself. Should I be looking at having the oil pump replaced or is this normal for a car with this many miles? Also if the oil pump needs to be replaced how hard is that? Thanks.