My DT Swiss hub (actually an old Magura Hugi hub that is a DT Swiss OEM) has always been louder than just about anything on the market. When I would get it serviced, it quieted down for a few weeks but then went back to its noisy self. When I figured out how easy it was, this is a quick service that can be done quickly and often.
To begin with, you need to obviously take the wheel off the bike and remove the cassette. This is pretty straightforward, but you will need a lockring tool and a chain whip. Look here for details on how to do this: How to Replace a Cassette.
Once you have the cassette off, you will need to remove the hub body. It does just pop off, but you do need to put some leverage on it. I take a rag, place it over the hub body and pull it off. Be careful when doing this as there are pieces inside the body and if you pull too hard, when it pops off you could have parts flying across the garage. There is a seal on the hub and once that pops open, the parts can be easily slid off.
As you can see when you pull the hub body off, the springs are holding in the star ratchets. It is important to keep all of the parts in the right order because you will need to put them back in this order.
While the hub is dismantled, now is a good time to check for any issues with your axle. Having seen someone crack a 12mm through axle on the last ride, it makes sense to take a look at yours and inspect it for issues. The groove at the end is where the end caps clip on, so you will want to make sure that the groove is cleaned out, helping ensure that it all goes back together smoothly. Also take a moment to inspect your spokes. You can see damage from chains dropping into your wheel.
I had a problem getting the spacer off on mine. It should just slide off but mine was not going to budge. Don't use pliers or other metal tools as this can damage the wheel. I decided not to remove mine, but the next time it is in a shop I am sure they can take care of that. Because I could not get the spacer off I could not take the axle completely out.
Inside the actually hub body you will see a set of teeth. These hold the star ratchet. This all needs to be cleaned out from old grease and grime. A Q-tip works well for getting inside there to clean the ridges.
The freehub body, once off, also has a set of teeth, clean these the same way, with a Q-tip or a rag. Make sure you get all of the dirt and dust out from around the seal.
I like to lay out a rag and as I remove parts, place them on the rag in the order that they came off. Then you can clean each one individually and make sure that you keep the order for reassembly.
These are the star ratchets. When people talk about the "points of contact" these are the things they are talking about. The star ratchets attach to the hub shell and the freehub via the grooves along the edge, and hold each other by the teeth on the top surface. If you have not upgraded your ratchets, now is the best time. These are the upgraded ratchets. As you can see they have 36 points of contact. The old ones have around 12 or 18. The greater the number of points of contact, the shorter the freehub has to turn to engage, so you have quicker response when you go from coasting to engaging. Also, it appears to have more strength. It is a cheap upgrade that is well worth it.
Once you have cleaned all of the parts, put them back together in the reverse order. Apply a good coating of grease to each piece as you go. I gave the springs a little "pull" to stretch them out a bit. This should give you better engagement by putting a little more pressure on the ratchets , pushing them together better. Make sure you have a good layer of grease on the tops of the star ratchets as these are the pieces that make all of the noise when you are coasting. My bike went from "swarm of bees" to silent after this servicing.
Once you have the hub all back together, give it a couple of spins to ensure that everything is working properly, then put the cassette back on. Remount the wheel on the bike and you are ready to go.
Two tips that came in after I wrote this:
From Jim Hix: If you leave the cassette on with the lock ring finger tight, it is easier to pop the hub body off.
From Kyle Poole: You can also remove the cassette, then place a quarter under the lockring, and screw that back onto the freehub. The quarter will press against the axle and pop the freehub off.