Rattling, grinding and vibration noise under the car.

my car is rattling and vibrating when idling then when l am driving it there is grinding noise driiii driiii noise. this is my first time to own this type of a car but it doesn't sound right. l took it to a garage and they said yes there are noises but they don't know where exactly the noises are from. they said they think its something to do with the transmission. Could anyone suggest what could be the problem please