Does it happen when the radiator cooling fan kicks in?

No it is just when we were sitting in busy traffic and the car wasnt moving much, after 2-3 minutes there is a intermittent vibration which can even be felt under both passenger and driver seat

Thats usually when it comes on in traffic when engine temp gets too hot

On some engines, usually small ones, when the radiator cooling fan comes on it puts extra draw on alternator, the engine slows down slightly and causes a vibration through car

Normally the ecu senses it happening and alters the revs accordingly but not always

I maybe totally wrong but just a thought....

Next time it happens, check to see on rev counter if the engine revs drop slightly,this will be when fan starts

and when it stops whether they go back to normal, this will be when fan stops

If it happens I think you will find its alternator drag on engine, and nothing unusual.