94 Mustang making tic tic tic noise after idling for 2-3 min

The car is a ford mustang, stick shift, has 240 000 on it, no leaks i've noticed. But when I left the car idle one day for about 2-3 min I came back and there was a tic tic tic tic tic noise. When I give the car gas the noise seems to go faster, the noise comes and goes, it came once when I was pushing the engine a little hard, but now it sounds normal again. Im told it could be either a heat shield from the exuast or some one mentioned a bad lifter? :(