What a rattle

I've never heard anything like this before. I'm only 20 but i've worked on a lot of cars in my day. It sounds like a can of bolts around the belt area, around all the pulleys and stuff. Drove yesterday with noise after sitting it for 2 months and the noise is terrible but seems as if I'm really not losing any power to engine. Numerous suggestions i've heard are water pump, harmonic balancer, a/c pulley, a/c compressor, too many things, someone please help me pinpoint the answer. No Cadillac should have to sit in the garage.

First guess would be the belt tensioner. Put a 1/2" breaker bar in the slot and relieve the tension on the belt. Then spin the pulley and listen.

What year and engine is in this Deville?

I have a 1991 Cadillac Sedan Deville. 4.9L V8