lol, there is an artery there that will pulsate as blood's being pumped through it. I feel that, too, and I always find it very fast on the doppler since my baby likes to hang out right next to it.
SCARED me the first time, though... I thought my baby's heart rate was 80 BPM... Then I moved over just 2 centimeters, and there it was, a 150 or so BPM heart rate... Ahh, much better. Hehehe