One of them annoying rattles

Hi guys, seem to have discovered an annoying rattle coming from the drivers footwell area on the right hand side on my monte. It only happens when idling in neutral but as soon as I touch the pedal and go to 1k rpm it disappears. I've also noticed it goes when I turn the air con on but soon as I switch it off it comes back... I've tried looking under the bonnet but can't seem to see anything that's loose. I have tried recording but it's not loud enough to hear. Any ideas? Thanks.