My check engine light came on abruptly and stayed on for about 500 miles, then went out. Now, when I start the car, it makes a very loud scratching/wheezing sound, and this lasts until the car gets over 20 m.p.h. Could this possibly be an O2 sensor problem?

 

Never heard of o2 sensor making noises...did you happen to check your codes while the CEL was on? Maybe check them with the CEL off? It's hard to diagnose, you have to be more specific. But if i had to guess...maybe a belt?