My car's making a weird sound 1975 300D

[fstop]

My 1975 300D started making this noise about 2 weeks ago.
You'll notice it goes away any time the engine revs up. I only hear it when the car is standing and idling or when driving at low RPMs.

At first we thought it might be a loose exhaust pipe/leak, but once we got under the car the noise seems to come from the transmission. Someone suggested it might be the torque converter.

Anyone have any other suggestions based on the sound clips below?

https://soundcloud.com/fstop760/car-sound-1
https://soundcloud.com/fstop760/car-sound-2