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With this sort of an issue, a good way to trace down such noises is to remove the belts and run the engine just briefly, then see if the noise has gone - if it has, then you know it is one of the driven pulleys at fault.
You can then check each individual pulley for play etc -manually feeling / rotating - tugging at the pulley wheels to reveal any issues and the same with AC compressor
pulley unit - -and that usually will if there is a fault there.. and then you can progress to a fix there.
If no such issues are found, then fit another, new belt - Sometimes its the belt that has hardened and is causing the noise- and that sorts it.
If the noise does not abate with the belt off - then the fault is either in the engine or the transmission
- which would need further/ deeper investigation....
BUt thats the way to go through it. Its time consuming so I suspect the dealer does not really want to get into that - but that would be the way. You will have to put this to the dealer thats looking at it for you -
Best Rgds --- MIKE.
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Rgds - MIKE...