noise at idle

Hi guys,
I have a '72 340 duster. I have recently noticed that my trans has been a bit noisy. While at idle I hear an erratic knocking noise. When i push in the clutch the noise will go away. I changed the gear oil. I put in 80w-90 lucas heavy duty conventional...it didn't stop the noise. The transmission has a little bit of trouble hooking up to 1st. Kinda bogs then will grab. All other gears shift fine. Any thoughts? Any input would be greatly appreciated.

-oldjunk