I recently changed the oil in my '01 Focus when I noticed that it was pretty low. After I was done I started my car and heard a sound that was brand new. The engine was making this low noise, almost like a diesel truck, a sort of knocking. A keep a good eye (or ear) on my engine sounds and I'm positive this wasn't occurring before the change. Once I heard this I kept it parked until I could get it to a shop. They checked it out and said they couldn't find a problem. They went ahead and change the timing belt but the noise persists. I know the oil change could be completely unrelated but it seemed odd. Any ideas as well estimates on repair would be much appreciated.
Also, the engine cap said that it required 5W-20. But I looked at a repair manual for that year and engine and it said to use 5W-30, so that's what I used. I don't know if that makes a difference.