Whining noise, and possible 02 sensor problem.
 

Sorry about repost, I didn't get any replies. I just want to get some help. Thanks! So I have a few problems with my blazer, first, I have a magnaflow cat, and flowmaster muffler. I have P0440 and P0420 codes, i read other stuff and sounds like I need to replace an 02 sensor, exhaust smells kinda bad outside, but isnt noticeable inside. I also have a whining noise when i first start it up. It is the same noise regardless of park, accel, neutral, etc. But after like 10 mins driving it goes away. I tried to find that on here and sounds like transfer case problem or bearing, but i wasnt sure b/c other threads didnt have anything about the sound going away after a little bit. Im gonna ask my dad when he gets home, he isnt a mechanic but is very good at fixing simple to minor things. Any help would be great! Thanks