What instrument do you find the most grating?
My vote would be for the calliope (if you don’t know what I’m referring to, think of the old time circus/carnival music that is both creepy and annoying). This morning when my radio came on there was a Stravinsky piece being played on that instrument, and it was one of those times where I started flailing around to turn it off as quickly as possible.
Bagpipes.
And bagpipes can be really nice sometimes. But when they’re bad, they’re horrible.
Mine, of course! Wanna see?! 
I’m pretty sure there’s been a similar thread in the past, but I can’t find it.
I’ll defend most instruments, including the usual suspects in such a discussion (bagpipes, harpsichord, recorder, piccolo, oboe…) Most of the dislikes come from hearing them played badly, or in the wrong acoustic environment, i.e. too small a room!
The butt trumpet.
There is something about an ocarina that creeps me out. I think it was that cheesy Mexican actor from the fifties who used to play one, what was his name?
The saxophone can be wonderful in the right hands, but there’s nothing so grating as a cheesy, easy-listening sax solo.
On the same train of thought, violin. Violin is beautiful. But when played by someone who doesn’t quite have the chops yet, it may be the worst sounding instrument in the world.
As for instruments that are annoying even when played well…the Jew/jaw harp kinda bugs the crap outta me. Never really got into the didgeridioo, either.
Have none of you ever heard the musical saw?
No, but I once saw a musical herd.
I love a lot of instruments that are often called annoying: banjo, bagpipes, harpsichord (I wish I could afford one), calliope, etc. But I’ve never heard a non-professional play an **acoustic guitar **and not want to smash it London Calling-style. It’s not the instrument, but rather the pretension and lack of skill with which its invariably played (ESPECIALLY here on campus). Is that cheating? If it is, sign me up for harmonica.
I vote saxophone, whether well or badly played. I just hate it. A pox on you, Adolphe Sax!
Right back atcha.
Have you not heard of the theremin?
I also want to mention I have a high tolerance for annoying instruments. I know how to play, reasonably well, the kazoo, banjo, ukulele, harmonica, theremin, and accordian. However, I want to destroy the harpischord. Forever.
Those are awesome. There was one in the movie Delicatessan, and it was great.
My vote goes to the hurdy-gurdy. Sounds like a cross between a badly played violin, badly played bagpipes, and a sack full of cats.
The oboe. Even when played well, it sounds like a sick duck trying to make love to an even sicker goose.
You want to be slightly out-of-focus while doing it?
I think the scene on the stairs in Animal House is more appropriate. It better captures the “STOP PLAYING THAT!” vibe.
Cartoon I saw once (Far Side?):
Sign in Heaven: “Welcome to Heaven. Here’s your harp”
Sign in Hell: “Welcome to Hell. Here’s your accordian”
Q: Why do bagpipers walk around while they play?
A: They’re trying to get away from the noise.