When Rumble, Hotheaded recycles a Ferrous Forerunner (a card unit) from the battlefield, does Karma, Channeler's ability trigger?

Ruling: Yes, Karma triggers when Rumble recycles Ferrous Forerunner because Ferrous Forerunner is a card being recycled to the main deck.

Nuances:

• Karma does NOT trigger when Rumble recycles mech tokens, because tokens are not cards (similar to how runes are not cards, as noted on Karma's text)
• Rumble CAN recycle token units as part of his ability cost, but this won't trigger Karma
• The distinction is that Karma specifically requires "a card" to be recycled, not just "a unit"