Vibration when air con turned to 'econ'

[skymaster]

When I tried to save a few pennies last weekend I turned the climate control to 'econ' ... upon doing so I get a slight vibration when idling in traffic... I can feel it coming up through the seat... A bit like you would get on a diesel with a dodgy engine mount...

When I put the air con back on it goes away...

Any ideas?