Ok, heres the problem.

Recently i notice when on a cold start, i get the moderate short squeak from the engine bay. Im assuming is one of the belts. The squeak only goes for like half of second and i will never hear it until its another cold start(like over night). While driving ill get no squeak or anything. 100,000km service was done 3-4months back. Also a regular service was done about 2weeks ago.

Any thoughts on how to diagnose/fix the problem?

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STi Version VI

if only on cold start more than likely will be the alternator belt give it a tighten see if that works

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slugeye gone

any tricks to tightening the alternator belt?

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STi Version VI

Starter motor might need a service.
Do you have a adjustable fuel reg? If so, they tend to make noise when you first start the car.

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11.1 @ 127mph

loosen the 12mm headed bolt that runs through the tensioner, just below the drive belt shield. Then tighten the 12mm bolt on the top, that is a long bolt that runs through the tensioner. Tighten it till the alt belt is tight (it will bring the alt up)

Quite common with new belts to loosen over the first few thousand k's

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Version 6 STI