Listen to the fellow who diagnosed a bad fan clutch. I believe the
electric fan should also run when the engine's temperature reaches 100
degrees C.

The good news is that the radiator is OK because the engine runs at 80
degrees C on the road, as it should.

The knocking you hear is preignition because the engine is too hot for
the fuel grade - are you buying premium? Probably not.

Assuming your aux fans are not coming on,  Here is a test .
There are two  auxilary  electric fans in front of the radiator.  With car
off, see if these turn freely by hand.- If OK >>>>>>
Next:
There is a 2 wire sensor on top of  the thermostat housing . It has a blue
base.
It turns the fans on to high speed at 105/107C.
With the car running, unplug this connector and see if the high speed  fans
come on. [ if yes , you have a bad sensor] If NO , the most common fault is the
fan feed fuse.
This fuse is not in the fuse box.  It is in the relay box behind the fuse box.
This will require  the removal of 6 phillips holding
screws for removal of the lid.
You will now see the relays . The one in Slot "C" is is the high fan relay and
the fuse will be on top of this relay.
Pull the fuse and check it.
While you are there , the next relay back
[ slot "B" ] should also be checked . This is the fuse/relay for the low speed
fan. This fuse was originaly 15 amp, but has been modifies by Benz  25, so do
that while you are in there.
If you found a bad fuse , relplace and  repeat the first test at the sensor
for fan operation. If OK,
take it for a ride ..and let us know ..
Don't use a/c on first test . as a/c is low fan operation on 124s.