engine putter's

[87 GT MUSTANG]

i was wondering if anybody might be able to help me out with a little problem i just seemed to notice today. I started my car up and it sounded like i had an exhaust leak, so i looked around the vehicle and noticed it was coming from in the engine. I popped the hood and it got louder ofcourse with the hood open and it sounds like the puttering sound is coming from inside the valve cover on the passenger side closest to the firewall. I was wondering if this could be a rocker or a lifter maybe and what could the potential be to fix it if need be, could it progress and get worse or should i just live with it. The engine is stock except for your basic bolt ons and i have 154000 km's on the car. My vehicle is a 1995 ford mustang gt. Also today i was under my vehicle and noticed that their was a pipe that comes out from the catylytic converter and goes all the way to the front of the vehicle is not connected, Could this be for the smog pump or possibly the egr valve or what exactly does it do, and with it not being connected what could it cause my vehicle to do, i notice i am getting bad gas milage as well so maybe this could be the culprit, i get about 250km's per tank if i am lucky. Any help would be greatly appreciated.