Does the Renn 575 hum / buzz like some of the other discs? Which discs hum the most? Thanks.
Yes, it does. All solid carbon discs make that roar. Which one does the most? No idea, haven’t tried them all.
depends how fast you are 
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Does the hum get annoying when riding a disc? It sounds cool when you pass someone on a disc, but it always gets me thinking that listening to that for hours on end would get old.
Nope, it never gets old, spurs you on really.
Does the hum get annoying when riding a disc? It sounds cool when you pass someone on a disc, but it always gets me thinking that listening to that for hours on end would get old.
You misunderstand the hum. The hum is all of your training behind you saying, “This is race day.” The hum is your bike saying “I’m here, I’m ready - the rest is up to you.” This sound a physical manifestation of everything that has gone into getting you here. The hum… oh, the hum… it’s not annoying. It is a gift. It is a reminder of why you are here. It is a reminder to your opponents that you are here, and you are fast.
I really like the hum. I never ride a disc except for races, and this is why. This sound gets me into a state of race pace intensity, so I’m ready what I’m going to do.
Yes, the Renn 575 has the typical disc roar (I like that term better than humm). I also agree that it is NOT annoying. However, you can use it to your advantage as others have mentioned. First as you “spin up” - you really hear it, giving you that psychological advantage. Then, there is the passing of other cyclists that “look back” to see what is coming 
Then, when/if YOU are passed by a non disc user, the psychological advantage switches back to "I’d better get going! I’VE got this disc and this guy on Ksyriums just passed me! Last: if going fast enough and a good, steady cadence, the roar dissipates as the wind noise in/under the aero helmet (a must if using a disc) now makes hearing the roar difficult.
x2 The rumble never gets old. The Renn rumble seems about the same as Zipp and HED. All “true” discs rumble much louder than covers or a trispoke.
Alright i’ll talk a little smack here. Yes the Renn makes the whoosh. You are asking if the disc noise gets old? Only if your getting passed by discs a lot. The noise that you here when riding a disc is less than what you here when being passed by a disc. Honestly.
I said “It sounds cool when you pass someone on a disc”. Not ‘… when someone passes you …’ Do you even hear it while riding one? It sounds neat, I guess I’ll need to borrow one and see how it is. I’m about to order a wheelbuilder cover as spending 10x+ for a disc is crazy (to me).
The noise is MUCH less noticeable to the rider/passer than to the observer/passee- i mostly hear it when riding next to a wall for the noise to bounce off of.
I said “It sounds cool when you pass someone on a disc”. Not ‘… when someone passes you …’ Do you even hear it while riding one? It sounds neat, I guess I’ll need to borrow one and see how it is. I’m about to order a wheelbuilder cover as spending 10x+ for a disc is crazy (to me).
It’s always fun to pass people with nicer equipment than you. It makes me smile every time. Sadly, the wheelbuilder cover doesn’t make the whoosh sound, but rattles around a little on rough roads. Small price to pay for saving $1000 or so and still getting (mostly) the same aerodynamic effect.
Yes, it does make that ‘sweet sound’, but like others have said it is much less noticeable when you are riding on the disc vs when someone else is riding next to you.
When I first got my Renn I was disappointed that I did not hear the sound on my first ride until I tilted my head sideways and then there was the sound.
Just as good, IMHO, as the woomp wooomp sound the disc makes is the sharp echoing crack or loud click made when shifting the rear derailleur. For whatever reason it seemed to amplify the sound of a good crisp shift for all to hear that you are coming to pass them. (Or if you are on a hill that you are bailing out and trying to grab any more gears to hang on!)
baxnelly has it right. The sound while on the bike is not really noticeable. You definitely hear it a bit more next a wall or something. And the sound of shifting is extremely loud with the disc.
As others mentioned, it does have the disk “roar”. Only really noticable to the rider when the sound has something to bounce off.
Another nice sound is the loud free body. Nothing like scaring someone when you sneak up and coast behind a few secs before going by with the roar.