This sounds awfully like badly worn transmission. The Dualis in automatic form used what’s called a CVT. Instead of gears inside the transmission, the CVT has pulleys and metal belts that vary their relationship and, therefore, create the various 'gear ratios' the car needs.

When these pulleys and belts wear, they can start to make the noise you’re reporting. And, just like your car, it will get noisier as time passes. Unfortunately, you’re probably looking at a transmission overhaul or rebuild. If you continue to drive it as it is, you’ll almost certainly do more damage, although it sounds like the real damage is already done.

Take the car to a transmission specialist who will be able to accurately diagnose the noise and take it from there.

In conventional automatic transmissions, a delay when selecting gears (often reverse) indicates worn internals and the need for a rebuild. Your Nissan is fitted with a CVT transmission rather than a conventional auto, but it still operates on the basis of hydraulic control. Which means that the problem could still be one of internal wear. A transmission specialist is likely to be your best point of contact for this problem.

The first thing to do, though, is check the level of fluid in the transmission. A level that is low can mean delays when selecting gears and even slipping of the transmission under load. However, if the level is low, there must be a leak somewhere as these are sealed systems and should not require regular topping up.

A car that continually breaks the same component time after time probably has a bigger problem than simply the part that keeps breaking. It sounds like you need to have the transmission and torque converter checked for alignment and correct interface. Any problem with the way they fit together could explain why an O-ring keeps failing. Continuing to replace the O-ring won’t fix a more complex problem.

Either that, or the mechanic who is changing the O-rings is making the same mistake over and over again and not using the correct lubrication or fitting procedure.