What is a rattling noise from the gearbox?
It might not be the gearbox. Does it sound like it's coming from below the gear lever? I had the same problem in mine, and it turned out to be the exhaust, which was loose and rattling. It was fixed by putting a hard rubber spacer between the pipe and the car.
There are a few things that might cause an air conditioner to make a rattling noise. It could be that the there is dirt and debris in the cabinet. If this is the case, cleaning the cabinet should stop the rattling. The reason could also be loose panels on the cabinets. The fan could also be bent and no longer straight.
When reverse gear is engaged on automatic gear car there seem to be noise coming from the gearbox what is the problem?
Transmission problems are serious...and if it isn't already, the noise is a warning that it will be very soon. There are any number of things it can be and it must be inspected quickly. (Of course one possibility to hope for...it isn't really the tranny but something like the exhaust rattling against it). Do not delay.