I am writing as I have made a coconut porter a couple of times now and I keep getting sour notes after I add the coconut to the secondary. I tasted the porter before I added the coconut and it was fine. I then toasted the coconut before I added it to the secondary
I was wondering if anyone might have any idea why this keeps happening?
Maybe it is acrid notes from the toasting process? You might let it sit out for a day after toasting.
I was thinking it might be the oil going rancid but coconut has mostly saturated fatty acids and as such its pretty stable.
Theres always natural coconut flavored extract, its a pretty good approximation.
You might want to try this method instead. I had a wonderful Coconut Porter from Flat Earth Brewing at last year’s NHC. They soaked the coconut in vodka for about a week, drained the vodka, mixed it with brown sugar, and boiled the mixture until it was had a gravy-like consistency. The mixture was added toward the end of the boil. Also use White Labs English Ale 002. It’s a very flocculant yeast that will leave a hint of sweetness that you’ll want.
You could always try this too:
http://www.amazon.com/Stirling-Gourmet-Coconut-Coffee-Flavoring/dp/B000LKZ3GK
I think the toasting would have sanitized it. How was it stored between toasting and adding it to the keg?
Have you had anyone else taste it? 
And welcome to the forum!