bad sound sounds like something is broken,loose,cost big money.depends on engine.start engine and take a look at pulley,most of them start shakin,later,before could be just strange noise

 

Just before the damper goes you will feel the car shaking and shuddering on idle, when it does go it will teal all the belts in most cases and damage most of the other pulleys...

Its worth replacing if your in doubt, the damper itself costs around 300 IIRC, if you let it go your looking closer to 600...

 

When mine went, it threw the A/C belt off first and I guess if i'd left it go for much longer it would have thrown the auxilary/altenator/waterpump belt as well. I priced them in BMW dealers €320 approx for part alone, so then went to OTTO in finglas and got one for €180 plus VAT approx same brand febi bilstein and I got BMW independent in Baldoyle to fit one hours labour.

 

Hi! can I just ask you what model of BMW this was?
The same thing has happened on my 02 E46 320D (150BHP model)
The mechanics were quoting me €265 originally, but then when they realised it was for the higher BHP model, the price jumped to €340!!
I phoned BMW and the price they gave me was near enough €340
However on looking from your post, I phoned OTTO and they quoted me around €180??!!!
The guy in OTTO seems to think that this is the part I'm looking for, i.e. the crankshaft pulley that suits the 150Bhp model!

 

problem fixed it was the cankshaft pulley, the damper in the middle had seperated and made it start rattling, new pulley bought and installed for E170, sweet!!

 

glad you got it sorted kormac, bad news for me is its after starting to make a knocking noise again