95 ram 2500 rwd/2wd 46RH Overdrive whines/growls. No noise with od off. ?

This thing has 300,000 miles on it. Shift quality is consistently good. The noise follows wheel speed, and appears ~1/2 second before od clutch engagement (seems to be at beginning of underlap). I replaced the clutch piston bearing last night with no effect whatsoever. I'm told this is a common problem with 46RH overdrives, but know one around here seems to be able to tell me what's failing, only that I'm on borrowed time. Anybody know what this is about?