Weird noise from exhaust on 2003 Oldsmobile Alero

I have a weird high-pitched noise coming from the exhaust/under car. I know the protective plate fell and is now resting on the exhaust itself, but the noise is mostly coming from the back end part. I can hear just as loud when inside or outside the car only when the engine is on.

Asked by for the 2003 Oldsmobile Alero
It's hard to say what is wrong without being able to experience the noise first hand or examine the exhaust. I wonder if its a heat shield or the exhaust has become pinched shut and restricted.
It looks like it's a heat shield under there (in the center of the car). But, I think it fell on the exhaust. I'm going to get it fixed this week, but this evening, it started making that continuous noise. Reverse, forward, or parked, when the engine is on, the noise was still the same, not louder or softer.