transmission knock

I have a 76 Duster 318 with a 3speed automatic. Today it started knocking loudly at idle in park an the knocking speeds up when I rev it gently. It continues in drive but goes away in reverse. My stethoscope is relatively quiet on the rear main, really loud on the bell housing (I know the front of an auto tyranny isn't really a bell housing), and then quieter again on the main body of the tyranny. I've been driving it for about 6 months and its always had a high speed (65 MPH+) vibration which seems common on he forum. Not sure if they're related. Any ideas would be appreciated.