Bicycle Mechanics - Annoying Noise

Got a question. My MTB has Deore components, 27 gears. When I am in gears 24 thru 27 I hear the chain rubbing on the front derailler (not 100% sure about this). I have had it tuned and told the LBS dude specifically. This is the 3rd time I have mentioned this before a tune up was done (2 different shops) and they all say they don't hear anything. I told the last guy, that my theory was that when they have it in the stand, there is no pressure on the pedals and that probably makes the difference on whether it can be heard or not. Am I crazy? How can I adjust this myself? I think your bike should not sound like that. Someone had mentioned adjusting the barrel adjusters, which one?, turn left or right? Advice would be very much appreciated.

BTW, I changed my own tire for the first time the other day. It was so easy! I wish someone would have shown me how before!!! I am so proud of me! :p

Does it happen only in one of the front chainrings, or all of them? Where is your rear brake mounted on the frame?

Only on the big one in front, but I usually don't use the other ones that often. I may have to get back to you on that. I mainly notice it in the hardest gears. As far as the rear brake it is behind the seat post on the top bar of the frame. (does that make any sense to you?)

If the chain is rubbing on the front der, you can probably see it while you ride. Also, if it's that easy to re-enact while riding, but doesn't happen on the stand, the LBS should test ride it.

And congrats on the tire change. Oh no, another mystique shattered!

I just rode tonight and it is rubbing just slightly on the front derailler. If you are on the bike looking down, it is rubbing on the left side. I have tried half clicking, clicking down a gear, or up a gear and it still rubs. Any suggestions on how to fix it?

Usually when the chain rubs against the front der in the 3 top gears it rubs against the RH side (outside). This can be fixed by unscrewing the H limit screw on the fr der, and possibly tightening the gear shift cable by unscrewing the barrel adjusters a bit. If it really is rubbing on the LH side (inside) you would have to do the opposite. See what Sheldon Brown has to say about it http://www.sheldonbrown.com/articles.html

So the chain rubs the left wall of the der cage, and it stops rubbing when you shift to larger cogs? Weird! The only thing that comes to mind is that perhaps the front der is mounted at the wrong angle, and needs to be rotated on the seat tube. I'm thinking that, at that wrong angle, it's fine with the chainline created by the big ring and the midrange cogs, but the straighter chainline created by the big ring and the smallest cogs is at odds with the off-kilter der.

Still, this doesn't 'splain why it's only happening under load. Could it be something as silly as something not entirely tight, creating a different chainline under load? A loose rear der? Slightly loose chainring(s)? Rear wheel itself "cocking" slightly under load?