Viscous fans

There's usually a little resistance, as the fluid drags slightly, but it should effectively windmill when cold and be quiet.

Bear with me here. One good trick I learnt, to check for correct functioning, is to get the engine warm (so the viscous is locked up) and then shut it off.

If the viscous is working, it should stop dead almost instantly (as it's locked up, so as the engine stops so does it).

Now, on that basis, if you can do the same when yours is cold - or it at least stops very quickly - I'd be inclined to think there's something wrong with it.

You can also try and feed it a newspaper. I've never done this, but a fan driven by a working viscous coupling should be relatively easily stopped with a rolled-up newspaper.

Thanks Lewis, I shall give that a try

If you confirm that it doesen't lock up when hot by pouring hot water or whatever over it then you know it's knackered and time to replace before hot weather. Rumbling viscous unit is normally a sign that bearings shot and that allows enough movement in seals to let fluid leak out.
Is there any side to side play when rocking the blades ?
If cooling isn't a problem you could try simply removing the fan for a short while and seeing if the noise is actually coming from that.
Another thought,seeing fan attached to water pump, is that pump bearings failing.

Paul H