Weird noise coming from engine.

[SimplyMatt]

If i rev the engine a little i get this weird rattling/grinding type noise, it does not do this if i am driving just when i put it in park or neutral and rev it. It was about 30 degrees outside so i don't know if it was just from the cold. Has anyone else had this issues?

[JRL]

Could be a heat shield
Known to rust and start to fall off, only to rest on the exhaust, so when you rev it, it will rattle.
Easy enough to see, either put it on a lift or jack it up and crawl under it and start to wiggle the shields.

The other thing it can be is an internal rattle in the CAT converter, again, easy enough to find that.
Easier on a lift with a help looking or listening for the noise while you rev the engine a bit.
Unfortunately this is not fixable and you would need a replacement soon