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I happened to drive to work today, needed to pickup the cylinder head for the Z car from the shop in Beverly this morning. On the ride home, with my superior from work, I noticed a throb to the car at idle with the clutch pedal down. Now, I had noticed I was low on gas leaving Cambridge about 1 hour/10 miles ago. That's right, one (1) hour. We had spent the last twenty-five minutes inching towards Swampscott from the Carroll Parkway/Lynn Shore Drive rotary. The throbbing seemed to die down as I released the clutch pedal and revved the engine. A____ mentioned, "Aren't you low on gas?" I said, "That's it, I'm about to run out of gas." I pulled over and parked the car by the Lynn/Swampscott Beach and we each walked home. We'll see if A____ ever accepts an offer for a ride again (we both had a 10min walk at that point). After supper I returned with a gallon of gas. Starting up the car, I noticed the same problem. A throb at idle, that got better at higher RPMs. I drove the seven tenths of a mile back to the house, noticing on the drive that I had less power than I was used too. I sat idling in the driveway thinking, "I hope this isn't that spark plug that I misthreaded and had to be helicoiled back in place." Opening the hood I was greeted with the sight of the engine rocking away. I had seen the engine run without a plug before, the engine was out of balance and vibrated similarly; all my plugs were in order. I also noticed the throbbing/pulsing/rocking noise was evident in the exhaust sound,.I didn't notice any exhaust noise from between the engine and the tailpipe. In neutral, revving the engine seemed to alleviate the pulse I felt (did not have an assistant confirm that the engine rocked less). * Bad engine mount? Then why the exhaust noise, and the loss of power? * A hole in the exhaust system? Then why the rocking engine? * Am I not firing on a cylinder? How can I confirm that? * How do I get to Pep Boys so that I can fix my other car?