Since the club is a 915 you are not under warranty anymore, so probably no replacement or discount program. You can either order a new shaft from an authorized Titleist retailer or buy a used Titleist driver for the shaft (910 -TS series are interchangeable) to get a new shaft to install in your driver.
Give your club a thorough inspection to make sure it's the shaft and not the head that is problem.

As Barry mentioned, a 915 is most certainly out of warranty.
But before buying a new shaft, inspecting the head for rattling and cracks would be a great idea, especially if the shaft is not showing any signs of damage, another suggestion is that if you have a friend who also plays Titleist, to borrow their shaft and test to see if the same sounds come from the club when you hit it.
If you deem that it is the shaft take a look around on your used sport equipment stores, such as play it again, not too long ago, I remember seeing 910 drivers for less than 50 bucks, not a bad deal if you are just buying for a donor shaft.
If all fails, maybe is time for an upgrade. the TS line is going as cheap as 199 in some used stores nowadays.
Best of luck.

If it is cracked below the ferrule, and into the head, it would affect performance. Are you still getting the same distance and ball flight? You can take it to a club repair shop and have them remove the head to inspect. If not cracked, the head can be filled with a type of resin and epoxied back In place.