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Message: [QUOTE="Kawidude, post: 247730, member: 18582"] I recently had a similar problem with my '97 XR400. It was making a really strange rattling sound that seemed to get worse as it warmed up. I thought it was the cam chain or the valves out of adjustment. After repeatedly checking each of those items, I discovered that it was a loose core in my FMF pipe. The center core loosened enough to move and vibrate a little. It was frustrating because it really sounded like it was coming from the engine and not the silencer. I'd suggested disassembling your silencer and checking it out. [/QUOTE]