The Dispatch | Expert | Scudo Hub

I did the listening thing using some ratchet extensions and 74 was more rumbly than the others  (note it sounds rumbly through the extensions, not the high pitched whine you hear otherwise).  This isn't the most sure fire way though as all sorts of rumbles can propagate through engines in all sorts of strange ways.  To be sure, take a photo of the belt routing, then put a 16mm socket on a big bar and turn the tensioner (74) rearwards from the top, fwds from below to take the tension off.  Either put a pin through the locking hole next to the tension checking marks, or just slip the belt off.  Take supreme care not to get your fingers between the slackened belt and a pulley - if the socket slips off the tensioner and it traps a finger under the belt you can expect extreme pain and a really difficult job getting the tension back off with one hand, particularly if you just dropped the socket!  Start the engine and hopefully your noise has gone, thus confirming that the problem is in this system.  Stop the engine and check all the pulleys, it should be quite obvious which one has worn bearings if it's noisy enough to hear when driving.  On mine, and probably most, it was the tensioner pulley which had a couple of mm play in it.  Check all the others in case there is more than one fault.  Remember that on pumps/alternators it could be something inside them that is causing the noise rather than the bearing.  These may be difficult to turn by hand so try using the belt.  Once diagnosed you can pop the belt back on to go down to the motor factors for whatever you need. 
Pretty much the worst outcome of ignoring this fault is that it wears to the point where a pulley falls off, the belt will follow it and then you will have really heavy steering, no aircon and your battery will stop charging.  But it's quite a big battery, so you should get home!  Wind the windows down to provide alternative cooling rather than putting the electric fan on - you will need cooling as you'll be sweating from hauling the steering wheel round at every bend!