Thanks a lot for answering.
Yes, I estimated the frequency. I measured it today with an RTA and you are correct.
What I find weird is that it's noticeably louder within this one rpm range (cca 3600 - 3900)...
Can this be "fixed" somehow? I heard that Renault used to install some kind of soundproofing material under the front hood of the car, and that you can still buy it as an option. Do you think this would help?
To me it seems that the vibration/sound is transferred through the chassis itself and I'm not sure if the soundproofing material under the hood would help much.