WoodsterSS

I have a Cannondale T700 touring bike and I recently changed out the 38c tires for Marathon Plus 28's to help speed my commute along. On one particularly smooth section of road I noticed a rhythmic thump, thump, thump. I can feel it all through the bike, though it seems less pronounced if I remove my hands from the bars. If I stand on the pedals I feel it strongly, so I assume it is a front end issue. At any rate, I reseated the tires and brought the wheels in for truing. After truing I could almost say the problem was more pronounced. I'm 180lbs and I have been running them at 100psi and even 90 a couple times, but I don't notice a change with pressure. I began to suspect the headset or hub, but I can't detect anything obvious, such as play when I hold the breaks and apply force to the bars. Two bike shops have failed to find anything, telling me the problem seems very subtle. Maybe I'm the princess and the pea or maybe I am just lucky to have a couple miles of asphalt that is smooth as a babies bum, but the thump is anything but subtle to me. Any ideas?