I love my characters; all of 'em. Some are easier to deal with than others, but bless their cotton socks, I love them all.
From time to time, each of us has to deal with that persnickety character who seems to stamp their foot and demand more time on the front burner instead of the back.
It's that maiden aunt who you were sure was only needed for two scenes, three at most, who now has put down her crocheting and is standing smack-dab in the middle (or next door) to the crime scene.
It's your female main character's quirky best friend who was supposed to be a shoulder for your main character to lean on, or take her to colourful bars and insist she swallow far too many drinks with little umbrellas in them. Now you've realized that actually this character is a far better match for your main character's Prince Charming than she is.
The word you're searching for is 'crap-doodle'.
It's that odd uncle (or whoever) that you simply put into the plot for comedic relief who is now spouting all sorts of wisdom and seems to know what's going on better than your detective.
Yes, it's all part of the writing process, but it's also a really good reason to bang your head against the wall.
As the veteran of more than a few head bruises, let me say...don't do it. Instead, take three steps away from your manuscript (and the wall, while we're at it) and look at the entire plot. This attention-grabbing character might just be doing you a huge favour.
In the past, these characters have made me discover plot holes and character inconsistencies I might not have tripped over for weeks. They've made me re-think plots. They've even...made me change endings.
However, they might also be jumping up and down demanding attention because they're attention-mongers.
If you've got one of these whistle-blowing characters, take a moment and listen to them. You might have to pat them on the head and send them on their way, but you might also have to shake their hand because they just saved your story.