Some tankless blowers are louder than others (look for it in the spec sheets- maybe it's there, usually not) and some walls are more soundproof than others. (A brick veneer outside a plywood sheathed 2x6/R20 wall is a lot more soundproof than an clapboard sided uninsulated 2x4 wall with no wall sheathing, just a sheet of tarpaper.) If you can easily hear traffic through the wall on that side you'll probably be able to hear the tankless running at full-fan/high-fire, but it's not usually bothersome levels of noise.
Clearly moving it away from the window would be the best bet for minimizing the uptake of exhaust. They're not silent, but not real screamers by any means.
I have an older Takagi tankless mounted in a basement about 10' laterally from my bed, vented directly out the side about 6' from my bedroom window. I can hear it if there is no wind, and there is no other noises (traffic, birds, etc). It's somewhat louder than the sound of the refrigerator coming on 2 rooms away in the house, but not by much. Were it not for the window I'd probably not be hearing it at all unless actively listening for it through the floor.
