Tonight while driving my Bonny (highway, 55-65mph), I noticed a fairly loud thump coming from the front passenger area. At first I thought my passenger was stomping his foot on the floorboard... that'* what it sounded like. He swore he wasn't and said it seemed to be from the engine compartment. The thump repeated every minute or so, and stopped when I turned off the A/C... leading me to believe it was the compressor kicking on and off. But it didn't sound right. Any similar experiences, or ideas of what to check?

check the clutch on the ac compressor

Check to see what the compressor is doing when it thumps. All of them will make a click noise when cycling on and off.

Will check... but this is definitely no 'click' as far as noises go. I thought my buddy was stomping his foot on the floor when I first heard it. Today I had the A/C on all day with no problems, and the other night I had it turned to "Max"... could there be a difference in compressor demand there, that would cause a problem?

i think there is a difference in compressor demand

The only thing MAX does is open up the recirculating air vents, so compressor demand should not change, unless the difference in air characteristics is freezing up your evaporator, or something like that.

Have you ever had the A/C charged or serviced before?

I have not had the A/C serviced yet. I just bought my SLE about 2 months ago, and didn't start regularly using the A/C until 1-2 weeks ago. I can live with the thumping if it'* not damaging anything... the air is still cold, and I have only heard the noise that one time. However if it is something that needs correction now, of course I'll consider that to avoid long-term problems.

It might be something with the dampers... I've heard them thump before, but not every minute.