Bicycle Mechanics - Help!!! Clicking in drive train

I've got a fairly new Trek Madone (purchased from EBay, so no LBS warranty coverage) with Dura-Ace components. Over the last few weeks, I've started hearing a clicking coming from somewhere in the drive train, but can't figure out exactly what the source is. It only happens when the chain is in one of the lowest (i.e. largest) 3 gears of the 10-speed cassette and it doesn't matter whether I'm using either the big or small gear on the crank, as it happens the same amount on either. Also, it only occurs when there's more tension on the chain, typically when climbing hills. The same gears on the flat seem to be OK unless I increase the tension by quickly accelerating to a higher speed. I've lubed the chain, checked the front and rear derailleur alignment, checked the chain for stretching, and made sure the chain isn't rubbing on anything. Because everything is fairly new, I can't imagine that anything on the bike is already worn out. Just this morning, I swapped the rear wheel (Race XXX Lite) + Dura-Ace cassette with a different (cheaper Race X Lite) wheel + 10-speed Ultegra cassette and I wasn't able to hear it, so I think I've convinced myself that it's either related to my wheel or the cassette, both of which are as new as the rest of the bike. The wheel seems to be reasonably true and I don't see anything obvious that looks to be out-of-whack on the cassette. Unfortunately, because it only happens when the chain is under tension, when I put it on a stand and use those same gears, it's completely quiet. Anyone have any suggestions???
Thanks.

-Dave

http://sheldonbrown.com/creaks.html

AND

http://www.parktool.com/repair/readhowto.asp?id=123

turn up your ipod and you won't hear the creaking anymore.

seriously though, it could be anything. chainring bolts, bottom bracket, crank/spindle interface, derailleur hanger, derailleur hanger bolt, pedals, rear axle/dropout interface, your cassette could be loose, the QR on the other wheel may not have been tight enough. it might not even be in your drivetrain. if you stand while climbing, it could be headset spacers creaking, or your fork end/front axle interface, could be either of the races in your headset, your stem bolts, faceplate bolts, could be your saddle or seatpost.

I recently had the same issue on my 2006 Madone 5.2. Drivetrain clicking that was only heard while in the lower gears (largest cogs). It turned out to be a slightly loose cassette lockring.
I had the same issue on my wife's Madone too with the same wheels. I've found that you have to tighten the lockring beyond the specs listed by Shimano for the Dura Ace wheels we have WH-7850-C24-CL otherwise you get the clicking while in the low gears.

I had the same thing, it was a slightly bend chainring hitting the chain off the front deraileur. I spent months trying to track it down