Sounds like an obvious infection. Ditch the plastic, tear down keg, and clean and sanitize everything. It is very hard to go wrong on a brown ale as it is very open to interpretation. I had a few in the past with black patent or roast barley that I didn't care for but 'twas still decent beer.
Mine is basically done. I will give it a few more days to clean up and then crash. Will probably hit the tap next weekend. Initial hydro taste was excellent. Very balanced, no real overpowering roast. I am also impressed with the hops. I have always wanted to come up with something very reminiscent of Fosters ESB and I think the BG would be a good fit for the late hop bill. I am not gonna fool with sourcing Pride of Ringwood.