I'm thinking about running a benign prankster, one that plays tricks on the party. Nothing to endanger them, just a little fun. 

Any ideas on how to go about it? Specifically, if I say that the party hears something, that's treading on the DM's toes and makes it obvious that it's me that's the source and that it's a prank. Any thoughts?

Be a bard. At a certain point, if you’re the only conscious party member, cast spare the dying on all of the ones not stable and then pass the DM a note saying you drew on their faces, Jigglypuff style.

Could play a generic sorcerer and not tell anyone your subclass and have the DM secretly roll on the wild magic table but it only happens to them. 

The artificer's Magical Tinkering, combined with the shenanigans of prestidigitation and thaumaturgy, could lead to endless opportunities for minor amusing-but-vexatious pranks.  You can play a PHB tiffle artificer to get Thaum and Presti on the same character and still have room for an offensive cantrip from level 1, if you want one.

 I was thinking the artificer actually. Doing things like using the devices as well!

But my question was more aimed towards how to present the prank to the players - it might fall a bit flat if I just declare that I'm pulling a prank on them (or effectively doing so)? If I say that the party hears a sound (the role of the DM, usually), that might make it weird rather than fun/funny.

The few pranks I've been involved in involve passing notes to the DM - preferably in secret but that depends on how much your group meta-games.

In a game involving my namesake character, Thoruk the Duck Slayer, the wizard used minor illusion to project a duck in front of Thoruk whilst he slept. He had spoken quietly to the DM so I, the player knew what was going on, but Thoruk didn't, and proceeded to burn a rage immediately and then destroy the camp, whilst everyone looked on laughing - except the ranger, who rolled invredibly poorly as well and also thought that it was a genuine duck. To this day, Thoruk fears the return of the Phase Duck.

I'd definitely avoid running the prank yourself - the DM might have anything lined up for you, and they might even consider you setting the prank up to be a cause to trigger it - if you go into the bushes to set somethign up, you might get jumped by an ambush!

Letting the DM in on the prank, and them making the party take perception checks as they hear rustles in the bushes, whilst the players are unaware that they are actually checking to see your character waving their hands as they use magic, will work way better. Most DM's like to be omnipotent - there shouldn't be much going on that the DM doesn't know about!

Trickery Cleric is sort of a given…however…

A Sorcerer has access to the “Subtle” metamagic, which would allow a prankster to cast their spells without verbal or somatic components.

“Catapult” a random object at a random party member?

”Who threw that!?”

”It was that guy over there!”

Honestly, the “Subtle” spell is the perfect mechanic for a prankster…cast your spells with impunity, and leave no trace back to yourself.

Another one is to hide a “Darkness” spell inside an object your party members own…put it in their wardrobe, their breakfast, their backpack…when they open it, they will become engulfed in magical darkness, unable to see.

No harm; only pandemonium.

And never underestimate the value of a properly-timed “Feign Death” spell.

Essentially you just have to set them up with the DM ahead of time. It's the only way to ensure that the rest of the PCs don't immediately know that you're behind it. I'd suggest doing something like this:

1. Design a harmless prank
2. Inform the DM about the prank idea
3. Have a key word or activity you perform that the DM can't miss that will trigger the prank beginning
4. The DM will then effectively 'run' the prank for you
5. The other PCs play through it, unaware that you caused whatever it is until the right moment

Example:

1. Draft a love note from an undesirable NPC, which will be one of a series of love notes they receive, getting more and more absurd
2. Tell the DM about it, pass them the note
3. The trigger for it is that you go to collect any mail from the tavern keeper who receives it for you
4. The DM tells the pranked PC that there is a green envelope for them, and reads out the contents
5. Prank goes however it goes

1+ on Sanvael's way of doing it (much more eloquently worded than mine, but the same gist).

The purpose of fun pranks is to actually prank the players as much as their characters. They need to be unsure whether this event is part of the DM's plans, or someone else interfering!

We should draw a distinction here that what you are asking about is pranking the players, not just the characters. You can totally prank the characters just by saying what you're doing out loud and having the players play their character as if they don't know what's going on. I'd suggest doing this kind of thing as a supplement to your "real" pranks as they are much easier to carry out and it allows players to buy in to the bit. I've known plenty of players that absolutely enjoy playing the oblivious stooge that falls for prankster's tricks every time, and being upfront about it is a lot less likely to actually annoy the players IRL (which happens often enough that plenty of DMs have outlawed gnome as a race altogether as they are associated with this kind of thing).