Leveling Out a Decaying Bell Tone Level?

What is the best way to level out a tone from a single bell strike? Clearly the level reduces over a pretty short period of time (about 10 seconds) to zero. But I need to use that tone for a full minute or longer.

I tried cutting out (at the zero crossings) a fairly even section in the middle where the slope of level reduction was minimal. then I tried looping that in multitrack view. But even though it sounded totally smooth at the transitions when looping it with the repeat mode in edit view, when I looped it in multitrack, there was a noticable little high-pitched sound at the end/start points. 

Is there a better way to do this? Maybe flatten out the tone level and extend it somehow?