Everything has a beat.
From the strict cadence of a marching band, to "There once was a man from Nantucket," to the notes a robin sings, to the laps of waves against the rocks. Everything.
I once had a chow-chow who barked in a series of five: bark-bark-bark . . . bark-bark. Repeat. Bless his heart, I thought he was OCD until I listened to other chow-chows bark. Most of the time, they barked that same way, so I'm guessing it's a breed-specific thing.
Our writing has a beat, too. You know how that beat should sound to you. You hear it in your mind when you put the words on the screen. Sometimes, through the various editing passes we make, that beat gets skewed.
That's why it's important to read it one last time.
It's preferable to read it aloud because what your ears hear and what your mind tells you it hears are often two distinct things. And this can go either way. "Did I write that?" can be said with awesome respect as well as with abject horror.
If you can't or won't read aloud for one reason or another, at least read slowly on this read-through. Don't read for anything except the way things sound--words, sentences, paragraphs. The way the words, sentences, and paragraphs flow.
This is your beat. Your very own personal beat. It's got PROPERTY OF ME stamped on the front.
If something is off as you read, fix it. I suggest fixing it then and there and reread in the same way until you're satisfied. If you're not, your readers won't be either. Then continue.
And the breed-specific thing?
That's our genre.
A mystery shouldn't sound like chick-lit . . . unless it's a chick-lit mystery, of course.