Rumblings From Beneath appears on the Merchants' Guild Board in the City of Amaranthine. It becomes available when you have unlocked the entrance to the Smuggler's Cove at the back of the Crown and Lion Inn.
Walkthrough
Note: This quest is only obtainable after you finish Smuggler's Run. Completing Law and Order makes this quest unobtainable.
Once you have received the quest from the Merchants' Board, go to the Crown and Lion Inn and enter the passageway to the Smuggler's Cove.
You will be confronted by a group of darkspawn - four genlocks and a genlock emissary. Dispatch these and the quest will be updated.
Return to Kendrick to claim your reward.
Result
The darkspawn will drop random basic loot.