GM issued a service bulletin for the alternators on Astras with automatic transmissions to address squeaking (and maybe premature wear on the accessory belt & components).

Bulletin No.: 10-06-01-001
Date: January 11, 2010
Subject: Chirp Noise from Accessory Drive Belt with A/C Compressor On (Replace Generator Pulley)

This is to upgrade the alternator's steel pulley with a modern clutch-pulley. I suspect this was not installed at the factory to save money. We DIYed the clutch pulley install. This upgrade stopped all squeaking on our Astra (the accessory belt and tensioner seem to last a lot longer too). The clutch-pulley is a nice addition.

Below are links to the TSB (includes instructions and pictures). Plus a brief primer on alternator clutch-pullies (aka overrun pulley).

https://charm.li/Saturn/2009/Astra%2...%20A%2FC%20ON/

https://www.gatestechzone.com/en/pro...function-tests