I just replaced the two power tubes in my Orange Rocker 30H from the stock chinese EL84 to JJ E84L. When I turned the gain channel up, it sounded like there was a kind of bird whisteling when I wasn't playing. (Don't think there was anything on the clean channel.) I haven't tried the tapping test to check for microphinics, but is that a likely cause? (I don't have the amp here at home, so I can't test out things right now.