Why is it making that noise?

My friend has a 2002 Subaru WRX STI that has some mods, such as a aftermarket turbocharger w/ BOV intercooler co2 cooling system for intercooler and I believe maybe a cat back exhaust. So recently he ran it w/o muffler for awhile because the hanger broke but now it's back on so now it sound like a lawn mower and it quieter than it used to be. So my question is what could cause that to happen?