General Cycling Discussion - Down Tube Creaking

Help: I have a Giant Defy Advanced carbon bike that has a godawful creaking sound coming from the seat post/down tube area. When I stand on the pedals to sprint it goes away, but when I'm on the seat and grinding away it woulds like I am walking across a 100 year old wooden floor. Skreeek, groonk, kruuuunch, skreek. Nobody wants me in their pace line. The bike shop recommended reassembling the seat post with carbon assembly paste. The stuff feels like pumice in Vaseline, and it hasn't helped. When the bike was assembled with just grease, the seat post dropped an eighth of an inch each time I hit a bad rut.

How can I get rind of the horrible creaking and still keep my seat height fixed while riding?

Two suggestion:

1. Re-post this in Bicycle Mechanics subforum for more and better responses
2. Have you tried swapping in another saddle? Creaking noises when seated but not when standing suggests there is something wrong with your seat/setpost assembly. THe seat itself is probably the most common culprit in these scenarios.

Check the saddle to seat post connection. For that matter check the saddle rails.

Check the saddle to seat post connection. For that matter check the saddle rails.

I'll bet tightening the post/saddle rail connection will solve the problem. Try removing the saddle, greasing the clamp where it touches the rails and reinstalling.
Bikes are notorious for "telegraphing" noises around the frame, and things aren't always what they seem. I once spent a Saturday chasing an intermittent ticking noise from the bottom bracket/crank. Tore everything down, repacked, torqued, didn't help. Next day I was riding with a friend and asked him to see if he could pinpoint it. Turned out to be the tip of my shoelace hitting the crankarm every now and then.

Most likely the seat rails.

Thanks to both of you. I hadn't checked the seat rails. That will happen while the Thanksgiving dinner is in the oven. With a bit of luck, I'll be able to work off my big meal with a quiet bike ride. Just to be sure, I re-posted in the maintenance area of the forum. I really want this nasty sound to go away!

You may want to re-visit the carbon seatpost and re-apply carbon paste. That "gritiness" is necessary to keep the carbon post from sliding around and it is not as rough as you feel it is, the grit is actually roundish and not at all jagged.
If your rails are carbon fiber, it is important to use carbon paste as well.

Do you have a saddle bag? I realized that the horrible squeaking i heard on my bike, (I thought was from my saddle), was actually the bag rubbing against the rear reflector on the seat post.