164L rumbles at hot idle?

[Rodosta@domain.elided]

I have a 91 164L that makes a deep rumbling noise, only evident when idling 
at a normal to warm temp.  The sound is much louder when the car is on an 
incline, especially with the nose pointed down.  Any ideas?  
Without experience, my guess from the sound, would be a rear main bearing, or 
the transmission. 

Thanks,
Rick Rodosta
164L 122K mi
86 Veloce 90K mi

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