I have a noise that sounds like a bird chirping from behind the center a/c ducts, but it comes and goes and is not a function of whether I turn the heat - a/c on. The funny thing is that it seems that it comes on when I have a passenger in the car.

Where can I grease the bearings?

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'89 420 SEL
'90 300 SEL
'68 Olds 88 Convertible
'84 300 SD (sold it)