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Folks that have been around me and my 2002 KTM 400 would have heard me voice my concern about the amount of noise coming from the engine. I had lots of feedback that KTMs are notoriously noisy engine. I believe this is true, a little more clutch rattle, transmission noise, and counter balance knock.
There is a situation that happens that is not normal noise and requires maintenance - a loose primary nut. The primary nut holds the drive gear in place on the end of the crankshaft. For whatever reason, improper torquing or lack of threadlock, this nut can work loose. The primary gear then rattles, and can also wear the woodruff key and primary gear. When I inspected my engine, the nut was 1/8 turn loose and I was able to remove it by fingers only! The nut is aluminum and can not be retightened. Good news is the repair is inexpensive if caught in time. I only had to replace the nut and woodruff key.
A loose primary nut is a common occurrence on early model RFS engines, not sure if it also affects newer KTMs. I also found information this is common on other bikes such as DRZ400s and DR650s. If you have knocking on the clutch side of your engine that is not resolved by pulling in the clutch, check the primary gear.

Here's a link to the thread I found that helped me diagnosis the problem and perform the fix:

http://www.ktmforum.co.uk/technical-how ... attle.html

Thanks Mac, That is probably what I have on my 450, I probably do not need to sell it now

With this info from Mac (and I was one telling him his bike sounded just fine ... just like mine), I pulled the clutch cover last week. Nut was loose.

This weekend I was riding lots of gravel and trails and for the first time could actually hear the chain noise when travelling down the road. Wow! to the amount of noise reduction. Not to mention engine longevity. Easy enough fix.

Here is another link with info:
http://ktmtalk.com/index.php?showtopic=401572