The pollution has been spotted at a waterfall in the middle of Hawes which is popular with sightseers and photographers, according to the Local Democracy Reporting Service.
A spokesperson for the EA said: “Whilst pollution was visible, our officers were unable to establish the source.
“Water samples have been taken and our investigations are ongoing.”
Previous pollution in the beck was attributed to a faulty drainage system at the Wensleydale Creamery in Hawes.
Following discharge from the business in March, Saputo, which owns the cheesemaker, said the issue with the drainage system had been resolved and further remedial works would take place to ensure there were no future problems.
Regarding the latest reports of pollution, a spokesperson for Saputo UK said: “As members of the local community, we take environmental concerns extremely seriously.
“Following reports of pollution in the Gayle Beck, we have been conducting thorough investigations and found that the creamery is not the source of the oily substance which appears upstream of our facility.”