exhaust noise

my setup 10 s/c taco with stock manifolds and urd y pipe and trd cat back
today when i left work and accelerated a lil harder cause traffic was coming i could hear a metal buzzing noise sounded like it was in the exhaust. i turned on the next road and accel like before i wanna say around 2.5-3k. it was there again.(usually around that rpm range i wanna say.) next turn accel again and now its gone.
got home let truck sit and drove it was there for a few seconds then gone. no loose shields and everything tight. thinking its the slip joint in the urd pipe or maybe the muffler. any ideas?

 

Any one?