The two most common sources of strange mechanical noises on the XT660 motor are:

(1) Loose crank nut - RHS.

http://www.xt660.com/showthread.php?t=23951

(2) Loose decompression pin and/or incorrectly setup decompression mechanism - top end LHS.

http://www.xt660.com/showthread.php?t=24187


The noises generated by both these problems can resonate round the whole engine/frame area so are difficult sometimes to pinpoint and aren't always necessarily heard next to the source.

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