1998 Park Ave. below 700 rpm engine noise

I have a 1998 Buick Park Ave.Ultra with the Tthe 3800 Supercharged V-6 which has 63000 miles on it. I recently starting having a disturbing noice from the engine when I am at idle and below 700 rpm while in gear. The noise is gone when it is in park or in neutral. I recently replaced the oil pressure switch, but am worried that this is more of a transmission problem. The only trust worthy mechanic that I can find told me that it was blower noise. This is not a blower issue...

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