I have a 1997 GMC Sonoma with the 2

I have a 1997 GMC Sonoma with the 2.2 liter 4 cylinder engine. Couple months ago I started to notice that the truck when first started the oil pressure be around 50 to 55 and that once it warmed up it got down to 20. Well 1 day when I was getting some gas to head up north I had noticed a very loud rattle that sounded like a quarter and metal coffee can. I had stopped at gas station to check the oil and found out that it was 2 quarts low. Well after I had to put 2 quarts in the rattle kept on going. So I had drove it home and parked it. And a lot of people have told me it's just the oil pickup tube, because when it is idling and you give it gas, goes away. If it is not the oil pickup tube what could it be?