Well I've done a search here and found little on topic. Admittedly, I'm not a tranny guy. Here's my head scratcher... -slight rattle 1st and 2nd gear -rattle increases a little in 3rd -after you really get into it at highway speed, rattle gets pretty bad -shifts fine, no slipping -if I slip it into N while driving, rattle stops -i drove with the floorboard hump off and with a stethoscope found the noise to be coming mainly from the passenger side of center- close to the bell housing (not on the bell housing) I wouldn't think this would be a vacuum issue since it shifts fine. My best guess is some type of worn gearing. I've never torn apart a TH 350. I'm just seeking some direction before I yank this thing out and dive into it. Any tranny guys have any ideas? Thanks in advance.