Pros and cons of undercoating a vehicle?

[Forrest]

Undercoating is typically applied to protect against rust. On YouTube, Legit Street Cars (Alex) is based in Chicago and works on plenty of cars with rust. Check this clip YouTube at 50:35 mark for his recommendation

On a 16 year old car, I wonder if pin holes in the exhaust system or muffler is the true cause of engine noise.