I've never seen a viscous couple fail in a locked condition. I'm certain it
may happen, but in every instance of a failed couple that I've seen, the
couple failed in a mode that it never locked and the result was overheating
at low speeds, or no speed. In one instance, we repaired an overheat
condition by defeating the fan clutch -- we inserted a bolt that was longer
than the others so that the fan would be locked to the waterpump pulley.

Viscous couples are relatively new, and there were many decades of auto
production where there was no couple at all, the fan was simply bolted to
the water pump pulley and that was that. Viscous couples only lock the fan
when there is insufficient cool air to provide for the needs of the
radiator, the common failure mode is that the couple never locks the fan at
low speeds when temps are apt to rise. The default condition of a viscous
coupled fan is that the fan is not fixed to the pulley it is mounted on. the
fan is not connected to the pulley until heat rises sufficiently to cause
the viscous material to expand and lock the plates inside the coupler.

Since the default condition is unlocked, the common failure is that the
clutch never locks, and the result is heat rise.

I agree that the clutch can fail in a locked condition, but I reject your
assertion that this is common. I also submit that the vast majority of the
time, most people would not even notice a fan that was fixed solid to the
drive pulley. The purpose of the couple is to reduce drag on the engine, the
reduction of noise is an unintended consequence -- good consequence to be
sure, but a side benefit not the goal.

Back 25 years ago, when I was still trying to get into SWMBO's
knickers, she had a Triumph Spitfire.  She was on the M5 coming to see
me when the fan clutch let go and sent the whole damned thing
forwards, through the radiator.  Naturally, she noticed that all the
nasty noise had stopped, and the only difference now was that a silly
red light in the dash was lit.  She drove until the car slowed and
struggled and finally wouldn't go any more.  Then she had the AA tow
it to my house.

From the local scrappie, I got a Herald water pump with a fixed fan,
and a radiator.  With a hacksaw and a pop rivet gun, I fitted the
radiator.  The water pump went straight on and I crossed my fingers
when I cranked the starter.  It ran!

A week later, the head gasket went.  So, I put a new one in and
persuaded her to sell it.  We've been (mostly) happily living together
ever since.

I agree with Jack, except that I think the end has already arrived. The
previous owner already ingored the noise until it grew to a point where it
requires attention.

To the OP
Lift the car and move the wheel from top to bottom and left to right. It
ought to be solid, but it is going to move. Take the wheel off and then
remove the brake and caliper, then take the hub off and replace the
bearing(s). When you get to the brakes, you should see whare the noise is
coming from.