Harrum, interesting. Infinite scale on AD? That is a different concept, although luckily it's a slow enough scale as to avoid being overbearing. A toplaner most certainly: he can't afford to share farm or to try and get it under a duo lane's harassment, and jungling would be too slow at getting stacks up, and Diam's too vulnerable to ganks to have to sit mid and be open to twice as many gank paths.
His Q is at once his most valuable ability, but it is not outwardly overpowered thanks to the delay on its damage, which allows any number of failsafes to stop an execute or at least for the opponent to leave a grim memento. In a duel, with its increased damage, it's incredibly strong since it doubles as an escape for that sweet, sweet, bm drive-by kill. That said, its utility is of such a sort that Diam relies heavily on it, meaning counterplay revolves largely around making Diam waste it or use it incorrectly, since he's dead in the water if he goes in wrong. Much how Master Yi can be brought to his death for his misuse of Alpha Strike or Fizz is magnitudes more vulnerable when his Playful/Trickster is on cooldown. Like those abilities, too, it is his best means of farming, meaning if he falsely believes he's safe he might waste it on minions. Depending on his dash speed, it might also be easily dodged at the extreme of its range. So that's pretty kewl.
Regarding the W, I wasn't aware a lot of things were audible through Fog of War. I know certain things give global warning noises, like Corki's Package pick-up and the ults of champions like Kled, Sion, and Rek'Sai, but that's not really the same case. Perhaps certain things are audible if one happens to have the camera on a dark spot of the map, but it just seems unnecessary. It doesn't hurt anything it just seems like fluff, which Riot often tries to keep to a minimum, such as old Shen's Vorpal Blade inflicting Lifetap. The active is nothing special, just another ability to keep the target where you want them after you've spent your Q. That's okay though, you can't have every ability on an infinite-AD-scale kit be stellar. It can be considered a point of balance, like how Rakan's ludicrous mobility and potential team-wide Charm means his own Q, Gleaming Quill, is laughably bad. If it resets his attack timer the W can also have use as part of a burst combo or at least at making absolute certain you don't miss the cannon minion.
I feel the E could benefit from a little more clarification. For instance, is there a maximum Noise you can generate or is it, like his AD stacks, also without a cap? That's not really an issue of balance (it would be far from optimal to try and build high Noise stacks in the way you have them building here) so much as it is one of communicability. Champions like Tryndamere and Renekton who generate Fury have that Fury communicated in their resource bars, since they don't use Mana. Diam does use Mana, which means he'd need a third bar. And an actual one, too, not like the charge display for Jhin's and Annie's passives. Might be a little tricky to work into the display that way; especially if it is a capless resource (since, like, how big would you make the bar?). As for it's usefulness, I feel it would be harder than many think to activate the shield since players rarely stand still of their own accord. There are too may skillshots flying around all the time to be standing idle if you can help it; especially in proper fights. Even though there is a way to just stop, it would be a hard thing for many players, including pros, because everything else about the game rewards fancy feet, and stopping for even a fraction of a second can be more than long enough to eat a cheap shot from the Fog of War or suddenly not have enough time to clear an incoming AoE shot. The other active, the empowered attack, seems able to stack with W, which is actually pretty damn good. It makes Diam into a sort of laning assassin. He doesn't go sniffing for prey as much, since as a toplaner he's supposed to be making pressure away from his team, but if you wander in by yourself he'll Q through you, chuckle while informing you it's nothing personal, and then double-buff his next auto to remove the colors from your screen. A Slayer, like how Yasuo and Master Yi are, but more confined to his lane. The counterplay of course is that the combo is incredibly obvious (you should make sure the dash is slow enough to allow a reaction, by the way), so if he telegraphs it you can have all your wealth of dash-stopping CCs, autoattack cripples, ass-saving protections, and friends in the bush to teach Diam that silence is the loudest ragequit of all. Hey, it's nothing personal, kid.
Diam's ult is certainly interesting and unique, although again I feel some more communication on its capabilities wouldn't hurt. For instance, does Silence have a tether range to Diam (like how Tibbers and Shaclone do)? It very much better if it's loose from him for 12 seconds because the counterplay largely amounts to "kill the cocky prick for sitting there like he owns the place" and if Silence doesn't have a tether range then Diam can ult from under an inhibitor turret and, so long as Silence is a little faster than a Bootless Ornn, still have his sword show up at the outer turrets with a few seconds to remind the enemy laner how infinite stacking works. That said, the shorter you make the tether, the faster you can let Silence's movement speed be to compensate, since it's only fair Silence be able to actually do something if its immobile and undodging health bar is sitting ten paces from the very pissed-off enemy Kog'Maw. The longer the tether, the slower its speed, since it's only fair that that enemy Kog'Maw get into auto range on Diam's smug mug to have a fighting chance before Silence carves him into an Icathian Surprise with a side of salt. Don't forget to have a contingency in place for if Silence tries to Q out of the tether range. I know Silence is susceptible to CC (yo dog i heard u liek silences so i silenced ur silence), but since it doesn't auto and is invulnerable CC would be better spent on enemies that might actually die for it, and it actually becomes a boon to Silence by allowing it to intercept skillshots (since it has a hitbox) and bodyblock for its less-papery buddies. That's okay, since as a toplaner Diam is probably robbing the team of an actual tank, but CC'ing Silence is simply an extra mechanic for Diam to use, not a counterplay option in the grand scheme of things (12 seconds is long enough to eat most of the enemy team's CC chain, who shouldn't have thrown it at Silence anyway since there are four other people who would actually maybe die from it). Also, I'm assuming even though it isn't explicitly stated that recasting the ult for the dash also ends the ult early, just in case that bush Diam sat down next to wasn't as empty as he'd thought it was. And that's good, it's a little bit of safety for Diam since it is easier than one might think to make it past an Invulnerable flak cannon and kill someone who can't get away (ever dive an enemy squishy by running past an Ulted Kayle?). It gives Diam a little more of a failsafe. Also: does Silence share cooldowns with Diam? If not Diam might be a little too safe (Q me to safety, baby). But if so Diam might not be safe enough (I knew I shouldn't have used my only AoE to deal damage with my ult!). Maybe some kind of partial cooldown share? Hm. Does Silence share Noise with Diam? How would the Noise display work, would it float over Silence instead of Diam? What benefit would Silence ever obtain from the Noise shield, since that would be what triggers if Silence tries to E something while Rooted? It's a pretty cool and unique ultimate that gives Diam a good place in the League, but it just needs a little verification in there so we can all better imagine how it might interact in the many many strange exchanges of League. Does Silence activate items? What if Bard ults Diam but not Silence? What if Diam if CC'd instead of Silence? What if Silence is knocked away in such a fashion as to throw it against the tether range, like by a Poppy ult? What if Diam is hit by something like a Trundle pillar, Anivia wall, Ornn pillar, Taliyah wall, Azir wall, or similar "make terrain that someone obviously can't just sit inside" ability? Just some things that come to mind, is all. Editing some verification in there could help give us all a better picture of it.
In any case certainly a usable concept, just with a little more explanation added in for the more complex parts of the kit. I very much like it, and with the proper character in place Diam can be a unique addition to the cosmopolitan roster of League, although care must be taken to give his model a stance and movement of such a sort to clearly separate him from other one-sword fellas like Tryndamere and Yasuo at a glance. Being able to tell immediately who's standing there is something Riot has always been careful to communicate. It doesn't matter if Lee Sin's wearing a skin that has different stances and movements (besides his base, he's got three other animation sets I think) because there's only one barefisted punch guy with his body shape in the league. It doesn't matter if Blitzcrank's in one of his widely-different skins, the robot with two big arms could hardly be anyone else. Even if you've never seen a particular skin before, the movements communicate soon enough who's in the lane with you. To that end, make sure to make Diam stand apart from the ever-growing number of fever dreams that are League Champions.
Also: a joke/taunt for Taliyah/Malphite ("don't you know paper beats rock?") and Irelia/Camille ("Oh no. Scissors.") is damn near mandatory.
