It comes on as soon as the engine is started and makes loud hoover type noise when engine is revved, all the belts are back on properly.

There is also a weird noise at about 2.5-4k rpm from that area, sounds like a shook beer can full of tiny stones/bolts.

Also as soon as the fan comes on it feels as if it's holding the engine back almost like the engine goes down on one cyl.

When the engine is switched off the fan spins freely. None of this happened before the cambelt change.

Any ideas?

the fan has a clutch and gearing on it

 

does it run ok if you take the fan off? ie just bolt the fan pully on without the fan, just run it for a couple of min and see if that helps

 

I dont think its your fan tho, maybe a pully jamming, alternator etc

>> It comes on as soon as the engine is started and makes loud hoover type noise when engine is revved,

 

This is absolutely normal and this is one sign of how a good working viscous fan can be recognized.

 

>> There is also a weird noise at about 2.5-4k rpm from that area, sounds like a shook beer can full of tiny stones/bolts.

 

This sounds more worrying ...could be detonation.

Did you check/set the ignition timing after the installation of the timingbelt ?

 

> Also as soon as the fan comes on it feels as if it's holding the engine back almost like the engine goes down on one cyl.

 

In most cases the fan comes "on" around an engine temp of between 90-95 Celcius.

It should not hold back the engine, not noticeable anyway, so the holding back could be something else....overheating maybe ?