I haven't noticed it on my Nexus (because I rarely have anything playing), but my iOS devices do that briefly when email is syncing. Is it possible that something might be syncing in the background when that happens?
EDIT: oops, never mind. I didn't see that last part about it staying like that until a reboot.
FWIW, I had to replace my first one because one of the speakers stopped working after two weeks. Apparently, speaker issues are not uncommon with the new model.