Are return air grilles noisy?

We are getting a daikin reverse cycle installed the week after next and they came out today to check the roof cavity. Where they originally wanted to put the return air grille, which was in the middle of the back hallway, they can't because there is a bunch of water pipes there, so they are moving it up a bit and relocating a down light which is fine, but it's pretty much just outside of two of my sons rooms.

The kids sleep through anything, so a bit of humming doesn't bother me, but how noisy do they get?

Our return air is located in the hallway outside our bedroom and we sleep with the door open. The outlet inside the room generates more noise than the return air and both are barely noticeable.

This may vary depending on your system size, components and settings. We chose a generously sized return air and specified acoustic ducting for the return air although it probably wasn't required. Very short distances between the return air and internal unit may generate higher noise levels.

Most Daikin units I've seen are very quiet but if noise is a problem when you move in the return air and associated ducting could be altered to reduce the impact.