engine "noise" at 2000 rpm only?

[Vitalie]

I got the mechanic doctorscope and listened different parts on the engine. It is actually pretty cool.

The noise is coming from the metal pipe that connects the header and the TWC . I think they call it collector. And it seems that the noise is stronger at the header fitting. I wanted to take it out and inspect it as I don't see anything wrong from the outside but those nasty bolts on springs won't budge, not even after soking them in w40. I did not want to snap them or hurt myself as I am limited in tools and was using just the regular 14mm wrench.

What suggestions are there?

Thanks