A knocking sound from a wheel even when coasting could just be a tire...
But check that all your lug nuts/bolts are evenly torqued. Thump the tread
of your tires with a hammer, do you get the same note as the knock? See if
you can determine which wheel is making the knock, then rotate the tires
and see if the sound moves with the tire...
If it stays with the hub, then suspect a CV in the rear, or wonky bearing
in any corner, although bearings usually growl, not knock...  I'm still
expecting it's a herniating tire, My van has a knock on one of it's summer
tires... I don't let it bother me because my van won't be going anywhere
more than 50' from my house all summer, and I've got the winter tires and
rims in the garage in case I need to get it somewhere that involves public
highways.
I have heard that reversing a tire's rotation (Moving a used tire from the
left side to the right side so that it has to get used to running in the
opposite rotation) can cause the belts to develop a bit of play and that's
what the knocking sound is from...  I dunno if I believe it, but still,
make sure to keep your tires on their happy side of the vehicle.
I hope it's easy to find and that it's either a warrantied or a very old tire.
Take care.