Engine is louder than usual?

I have a 90 Lumina 3.1. It has a new exhaust from the flex pipe back to the muffler, new spark plugs and wires, new coil packs, new IAC, fuel pressure regulator and gaskets. My question is: Why am I getting bad gas mileage and have a loud engine? I used to get around 25 mpg. I now get 16 mpg. My pressure regulator went and had that replaced and it doesnt help. My engine is super loud when taking off.. similiar to a jet (not as loud as). It doesnt sound like an exhaust leak though. Exhaust Mainfold? How do I check for this? My engine has a vibration and no CEL.