Can get back to cycling now, I have fixed the car.
Awful rattle that was similar to if not the same to a problem I had before and my local BMW independent garage fixed it by replacing the water pump plus they changed the alternator belt as well as it showed signs of age (don't we all).
That therefore was my first port of call.
The water pump is driven by a spring loaded idler pulley direct off of the crankshaft pulley and when I disengaged the spring on this idler pulley into the service position the noise went away so it was either the idler pulley (also known as a friction wheel) or the water pump.
The former was almost half the price of the latter so thats where I went first and it cost just short of £90.
To change it you have to drop the engine at one end to get access to one of the attachment bolts via the wheel arch so you have to strip the liner out of the wheel arch as well.
That all done I put everything back together which in total took me 3 hours.
Well the noise was still there so it was back to the BMW garage for a £150 water pump.
This again requires dropping the engine and 3 hours later it was back together and time to run the engine to bleed the refilled coolant system but blow me the noise was still there.
Our daughter came home for a visit from Friday night to Saturday night so my car time was taken up with socialising and shopping etc.
Got back under the car this afternoon (Sunday) to take bits off and inspect them.
According to the book the alternator belt tensioner should be quite easy to push up and lock into service position to get the belt off but mine required a bit of leverage and once it was moving it was then pretty easy to lock it with the service catch.
I had bought a new belt on Saturday just in case I found mine damaged but the old one looked OK apart from one small crack.
As I had a new one (£18) I fitted it anyway on a stitch in time basis.
I also removed the crankshaft pulley to inspect it and the engine casing behind it but found no faults.
Anyway put it all together and started the car and blow me the blimming rattle was gone.
My theory is that the alternator tensioner pulley arm had locked into a certain position and when the water pump drive pulley was engaged it just slightly changed the tension on the alternator belt, which is rests on where it wraps around the crankshaft pulley, and caused the pulley on the alternator tensioner to rattle.
I also have a sneaky feeling that this was the problem two years ago so have now paid over £300 pounds on parts for this fault over the past two years when all along it was the alternator belt tensioner which I will now replace at my leisure.
Anyway as a result I may not be richer but I am wiser.
I will probably have to
Ebay my old bits too recoup some funds as the car fixing has blown my new BKool Pro budget.
Still need two new front tyres as well and have a four wheel track and alignment check done as my wife hit a big pot hole just before a roundabout on a dual carriageway on the A303 (at Podimore just outside of Ilchester near Yeovil) on her way to the Bath and West.
Need to put my stock spings back on as the lowered Bilstein Springs I fitted a while back are a bit harsh for everyday driving on UK roads, especially with all of the speed humps in town.