Noise from rearwheel single-speed

http://www.youtube.com/watch?v=fik7azypOKU&feature=youtu.be

Is this normal? If not, any ideal what could be the problem?

Isn't it the bearings?

Looks and sounds like one of those Dicta freewheels.They go like after it rains.

lube the freewheel

...or something thicker like engine oil is fine in my experience

the axle bearings sound fine, but if you're really bothered by that noise you could repack them http://sheldonbrown.com/tooltips/hubs.html
make sure you put in 'plenty grease', will make placing the balls into the race easier, cut down friction and keep the shite out

Put the wheel on it's side, freewheel side up, spray a bunch of gt85 or similar into the gaps on the front of the freewheel, rotate and repeat until the rusty gunk goes onto your floor. If you want you can then chuck a little light oil in there too or just give it another spray every few weeks.

I think the freewheel sounds normal - no noise comes from them when in 'drive'.
Wheel bearings sound dry and gritty - need a clean and grease, or replace if the cones look shot.

How does it feel when you spin the axle, smooth and easy to turn or difficult and bumpy?

Freewheels are fun to take apart and reassemble.
An a-pawling job if you're really pissed, though.

How does it feel when you spin the axle, smooth and easy to turn or difficult and bumpy?

very smooth, I had the bearings replaced and the noise is still there