"Clacking" noise coming from drivetrain I have a 2005 Specialized S-Works Tarmac with Dura-Ace components and Mavic Ksyrium SL wheelset.  

I get a "clacking" noise coming from the drivetrain area only when pedaling forward.  I say "clacking" because it's not a click or creak noise.  It reminds me of the noise those acrylic Clacker toys from the 70s made when they hit, but obviously not that loud.

If I backpedal the noise will go away temporarily.  I do notice if I backpedal, then pedal normally, the noise goes away; it will not typically return until I freewheel then pedal forward again.  The noise has been around for about 2 years on this bike and I have changed the following:  Cassette from 7800 to 7900 series, chain, 53T chainring and pedals from 7800 to 7900 series.  The issue is still there.  It happens in the small or large chainrings regardless.

The "clacking" noise does not happen freewheeling at all only pedaling.  It does not correspond with wheel revolutions or even crank revolutions.  It's more of a loud clack immediately followed by a smaller clack happening about every 3/4 crank revolutions.

Last time I took the bike in for a overhaul I told them about the noise that I thought it was coming from the bottom bracket that I wanted that rebuilt.  Once I got the bike back the clacking was gone for about a week, then returned.

Thanks for any help on what to look for!

Mike
I had a similar problem until recently, initially thought it may be the BB, but after cleaning, greasing, and eventually replacing the problem was still there.

Turns out it was the freewheel hub on the wheel, and fairly quick to fix, if that is your problem, open up the freewheel hub, I needed 2 5mm allen keys for this, but various wheels have different methods of opening up.  Don't force anything, these are easy to destroy, and expensive to replace.

Once you have the freewheel open, make sure that the little pins stay in place, most new hubs will have springs keeping them in place.  

Apply grease, I used the Park Tool one (personal preference), and close everything up again.