We have the same issue on our'01. It has been there since we got it early August. My opinion is that it is just vacuum booster noise and will have it checked at 1st service. If I notice any braking problems will take in immeadiately. So far seems to have no effect on system.
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'01 GG LX
'94 Ingegra GSR
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'01 GG LX
'94 Ingegra GSR