Are you sure they're palpitations? I found this article on
Livestrong and a few other websites that seem to indicate that heart palpitations are rare DURING exercise.
Sorry I can't be of more help. When I'm on the elliptical machine, I experience some loud/heavy heart pounding sensations in my chest. It's not pleasant, especially since I wonder whether it's normal or something I should see a doctor about. (Up to this point I've chalked it up to being newish to exercise - again - and my heart trying to catch up.)
I'm interested to read what others have to say.