I have a similar noise on my 300d. it has been there since I got it. Is yours rythmic at about 1 second intervals? mine changes interval if I let off the gas pedal - it will usually slow down to a longer interval, but the pitch does not change.
It is not flex discs for me, I already changed them. I have a new transmission mount and one new motor mount (long story). Next I am going to change the differential fluid on general principle, and then the center bearing, as soon as I crawl back underneath and figure out where it is.

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andy t
'78 300d
'95 volvo 850, wagon
'86 300sdl - engine out, maybe I'll have it rolling by June

whole bunch o' bicycles