First, just to be sure we are talking about the same thing, I wouldn't characterize the sound I hear as metallic but more like a gutteral, low pitched, muffled RRRRRRRRRRRRRRR for about approx a second. Since the engine is made to cut out, the vacuum pump must be driven by a dedicated electric motor.
Now of course any time we step on the brake we're using up vacuum from the reservoir. When we start the car or take it out of Park or just change from D to R or vice versa, we are using a substantial amount of vacuum. During these moments, I tend to step on the brake more deeply than, let's say, coming to a gentle stop. I hope this is what you are hearing.
I bought my 18 EX Premium at the end of January. I'm still learning it's little quirks that I suddenly become aware of here and there.
I must add a disclaimer that although I'm a trained mechanic I'm not a Kia technician. These are assumptions based on 40 plus years of turning wrenches on cars, aircraft, boats etc.
I know I'm right until I find out I'm not and so then I just learned something new.