I've been experiencing the same thing in my beers and also beers from other home brewers in my area. I always taste a faint plastic flavor that flattens out malt/grain flavors. It's not disgusting just sort of flattens out the taste. Reading has led me to believe that this is a chlorine or more specifically a chloramine issue. Water reports for our water supply indicate use of both.
From what I've read chlorine alone isn't a huge deal b/c it will boil off. Chloramine on the other hand will not. You have to filter it out.
Anyway, having asked around to some local homebrewers who have incredible beer using the same water. It seems a constant difference in our technique is that they are filtering the water. I believe that they are removing chloramine and that is what's making a difference.
I have recently purchased a filter from an online homebrew shop that claims to take care of this issue. I cannot confirm a difference as I have yet to use it. Also all of this information is from research from this forum. Take from this post what you will. I'm a beginner who is no expert. I just thought it might be helpful to share my own struggle with the same issue.
Best of luck, please post back what you find, I will try to do the same.