Addendum to "growling noise coming from Nissan Xterra"

This is the second part to a question I had about a sudden “growling” sort of noise coming from under the hood (I think) of my Xterra (2003 XE, 2WD, manual, 4L). It is loudedst when I’m in 2nd gear but it’s audible in all gears, happens when I accelerate, backs off when I back off the gas. No other changes. Just the noise.

So I went out to check under the hood, and all the fluid levels and hoses looked OK. However, on a smooth metal piece that covers the left side of the engine bloc (I think), there is a rusty crack…Ummmm…is this a big deal, or this is just a crack in something that protects the engine bloc and as long as its still basically protecting the engine, it’s OK? Does anyone know if this would produce the “growling” noise with acceleration? I checked the fluid levels and they’re all OK. Oil is OK as well. Muffler looks OK (I checked that b/c the noise is somewhat similar to a noise my old Honda Accord made when there was a rusted-out hole in it)…Anyone have any thoughts? Thanks so much for all the previous answers too!