I'm extremely happy to finally solve this issue and thought I would share. I've been annoyed by a loud, metal-against-metal noise coming from the drivetrain of my BD over the past weeks (months?). I spent most of this time trying to reduce chain bouncing since I had the impression that was the cause of this noise. This is obviously famous for BD and X-conversions alike since the chain is unusually long. Today I even considered buying an idler (the famous, somewhat overpriced terracycle idler kit). This is particularly hard since I'm out of the US so anything gets here very expensive. Anyways, I thought I'd give it a last try before moving on.
BTW, my prior attempts to reduce the noise include wrapping some areas up with old tubes (http://farm6.staticflickr.com/5348/9205615888_84d5306afe_b.jpg)
. That obviously helped with the (real) chain bouncing noise. Nonetheless, the real annoying, louder noise persisted every time I went downhill or hit bumps, potholes and stuff like that. I also increased the tension of the Deore rear derailleur that came stock with my BD (opened it up and moved the spring to a higher tension position). Obviously, the b-tension screw was already at its max tension position. Today I decided to remove extra links from the chain to make it shorter, therefore increasing the tension and reducing room for bouncing around. Apparently the BD comes with 2 extra inches of chain (according to Sheldon Brown slack for derailleur chains can be reduced down to large-large total length plus 1 link, so I cut one extra out). All this helps, but the worst component of the noise still persisted.
To make an even longer story short, today I finally realized where the noise came from and decided to share it in case someone else also suffers from this very annoying problem.
The loud noise coming from rear drivetrain was caused by the derailleur itself hitting the frame, close to the right-side dropout. This became obviously when I found marks on the derailleur cage compatible to several consecutive hits (http://farm4.staticflickr.com/3677/9205615076_ab1ffb91fc_b.jpg)
. Apparently this is known issue with Shimano derailleurs (particularly Deore components). Shimano even sells a rubberized protector to be installed on top of the derailleur in order to silence it.
So I decided to go the same route and installed a piece of some foamy, old yoga mat I had around on the frame (http://farm8.staticflickr.com/7303/9205616712_a20f90c590_b.jpg)
, on top of where the derailleur collides with it, and the noise is gone! I couldn't believe how easy it was after all to silence it.
Anyways, if you got a similar problem, give it a try. I know it's less of a problem for Xtracycle (FreeRad) converted bikes. I used to ride one for several years and never had a problem with chain hitting anything of derailleur noise. On the BD it was driving me nuts.
Keep on riding, radicals!