Chirping noise from 03 Tundra

Hello to all, I have a 03 Tundra access cab that is making a noise that is quite hard to explain, so be patient. It is coming from under the truck, and if Ihad to guess, I would say it is coming from the exhaust somewhere. I don't hear it all the time, and it seems to happen more often at 35-40 mph. Now, to try and explain the noise, I used to think that is was birds chirping in the trees and bushes that I was passing until I heard the noise in a parking lot passing between rows of cars. So, the noise is mechanical bird sound if you can imagine that at all. Could it be something in the manifold? any ideas before I take it to the dealership would be much appreciated. thanks DD