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Honda CRF250 strange noise, cam marks are not perfect in TDC.

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Hello everyone, I'm a new guy. :)
I recently bought my first 4 stroker CRF250 07.
I came up to some problems with it :/...
There's this strange sound that comes out from the engine when its on idle and on low rpms. Video:
In top dead center my cam marks do not perfectly aline. I tried comparing my Cam chain to a new OEM chain and it was only around 1-2mm more in length. Mechanic told me that it was still in good shape and didn't need a replacement.
I suspect that this sound might be the decompression valve engaging on low rpms. Because when I slightly put them up the noise goes away.

Might the 1-2mm difference in the chain make the cam marks not aline perfectly?
And how could I tell If my tentioner is working good? I tried pulling it out and the spring mechanicsm is still pushing real good.

Technical info:
Piston and crank shaft in good condition only got around 5-8hours on them. ( checked them myself)
Shimmed the valves, all in spec now.
Starts in first kick, runs great...

Any ideas?


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