Hi all I am new to this forum and this is my first post.  I have a 2006 Octavia 4x4 estate with a 2.0l BMM diesel engine.  At idle approximately 800rpm there is a loud resonance that can be heard inside the car from the engine bay area, however, with the bonnet lifted the noise cannot be heard.  If you raise the rpm to say approximately 900rpm the noise stops and does not reoccur at driving speeds.  I am a technician by trade so have a good idea what I'm looking at but this is frankly annoying now.  Has anyone had any experience of a similar issue?  Any advise appreciated.

Have you checked the bonnet fixings?