C3 and Leslie Problem

[Guest]

Hammond C3 from 1960. This Hammond has a Trek preamp and a 11 pin plug. I connected a 722 Leslie. Works fine no problems. Half moon switch changes the speed from fast to slow to off. There is a “y” adaptor to connect a second Leslie. I connected a 716 Leslie. Both Leslie's operate as they should except now the 722 has a noise as if there is a ground missing. The noise is not exceptionally loud but noticeable. What should I look for?

[David Anderson]

If both 716 and 722 are earth-grounded, you may have a ground loop through the earth grounds and the audio grounds.