Funny, I just got one as well, and was puzzled by exactly the same thing. In my case, I also have the momentary receiver, and when I tapped the key (most of the time) I got a momentary pulse on the output, although it was sometimes a little jittery.
Whenever I hold the button down, however, I get the same pulses at the receiver that the OP observed. To my eyes, it looks almost as if the keyfob has an auto-repeat function. The pulse rate is approximately 2Hz. I suppose I should check the battery, but my unit was ordered only last week.
AFI- does this unit, in fact, auto-repeat?
I will let you know how I make out.