Weird noise at certain rpm. Can they be trusted?

Hello guys i own a wr250x 2009(17000km) and have a problem. i heard a weird noise in engine on certain revs but i thought it was the front sprocket so I took it to my mechanic. They told me this is a problem that yamaha cross bikes have too(and that this is a industrial problem). This is because some part of engine is not well lubricated and the materials are wearing out. He showed me the cams cap and told me that the side, where the chain comes on, has bearing and the other side doesn’t, and that side should be the problem. They were telling me that they had to cut some materials out (I don't know much about bikes). Also they told me that this is a problem that will never go away, the sound will come back in certain time and they had to repair it again. And that this process can be used only 2-3 times then I will have to buy a new cylinder head. The price of that service will be around 520e( still waiting for correct numbers).
So pls help me, is this really possible? Is this true? Anyone had that problem?

My opinion on that:

Bunk.

Soryy bad at English. What does mean ''bunk''?

Bunk = Don't believe it.

When you say "I took it to my mechanic" what does that mean?

Do you have a personal relationship with a certified Yamaha motorcycle mechanic?

If I had to guess, I'd say you need to find a new mechanic.

No i took to a Honda dealer,( we have no relationship, he was just the best option in my city). We have a Yamaha dealer in my city but all are saying that he is the worse mechanic in the country.

Bring it to your Yamaha dealer and see what he says...

But I agree that the Honda dealer is bullshitting you.

I realy don't want to go there. He was my previous mechanic(on old bike). I ower payed him, hi did calculate thinks that even weren't done. My firend has warranty there and is driving his bike there and is not satisfied with him + the service hour is 10€ more expensive then elswhere. I will try a mechanic that my other friend suggested me, but he is like 30min away

i got correct numbers now.
They callculated me: cylinder seal 23,07€, head seal 40,59€, engine seal 20,09€, seal engine cover right 19,57€, seal engine cover left 20,09€, consumables materials + HANDLING tread CYLINDER AND HEAD + Turning (cutting) head 188,52€, OIL filter 3,7€, oil 15,5€, working hours (5,2h) 106,56€. All together is 438,55€ + tax 96,49€ so it is 535,04e
(I had to translate it to English so maybe something makes no sense) So what do you think on that?