Engine revs, brakes thump

Saturn SL 1999. 111,000 miles. My car has recently started thumping when I brake. It sounds like it is coming from the front driver's side. It only happens when I press the brakes moderately or softly, not when I brake faster. Also, my engine has started to rev when I start the car (~2500 rpm). It goes back down after 3-4 seconds. It also revs whenever I push the clutch to take it out of gear (it increases between 200 and 500 rpm). This also goes down after a couple seconds. It seems to be idling higher while it is in gear as well. This is most noticeable in first gear, it will pull the car at about 10 mph without my even pushing the gas. I have a third problem that is the muffler and exhaust pipe have been rattling sometimes lately (when idling and starting the car). Also, 3 or so times in the last few months I have thought I smelled gas when the car started but it went away quickly. It hasn't happened consistently and it wasn't very strong either. Should I be too concerned by this? Thanks a lot!