Belt wobble/squeal
 

Had a low rumble appear over the last few days aswell as a rattle/squeal noise.

I've narrowed it down to the belt that drives the power steering/air conditioning/aternator etc

It looks like the belt randomly gets loose and wobbles then seems to tension back up again and the noise goes.

A spray of belt slip cures the noise completely for a few mins.

Question is, how do you know if it's the belt or something else?

Tensioner maybe?

Could be a number of things, however if it derails or breaks it has a good chance of taking out the timing belt and killing the motor and it won't have mattered what the exact cause was. Top priority.
A likely suspect is the alternator freewheel pully. As they fail they can go into a twilight zone where they come and go. Seized they will cause the tensioner to move about a lot and the belt to flap. They can free up for a bit and then the tensioner settles down and the belt runs smooth. There may be cricket type noises when it's running smooth.

I'm just going to get it changed asap as a precaution.

How easy is it to do with average mechanic skills?

You need to check the belt tensioner & idle pulleys, alternator pulley, air con compressor pulley, power steering pulley for roughness, seizure or wear and replace any that are suspect along with a new belt.
It's not too difficult to change the tensioner & idler, the alternator pulley may be best done at an auto electricians workshop (saves buying the tools) but not difficult to get the alternator off, don't know about the power steering or air con compressor pulleys but obviously the systems will need draining or evacuating to remove the devices.

But it's the alternator pulley that can self destruct and take the engine out?

You don't say what engine is fitted - on the 5 cyl diesel the alternator freewheel pulley can be changed easily with the alternator in situ.

2006 2.4 d5

Check the crank pulley as well. I've had one fail where the outer and inner metal parts delaminated from the rubber shock material that connects the two.

99% certain to be the alternator pulley and as per other advice on here, you need to change it urgently or other damage will follow & that could be expensive engine damage.

So is the alternator pulley the real danger?

Why I an asking is, I plan to just change the pully and a new belt and see if the sound has gone. If it hasn't I can still be confident that area with all the pulleys etc won't cause my engine to ****e itself?