Leveling Out a Decaying Bell Tone Level?

[KenTheriot]

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? 

[Rag and Bone]

I'd just mix it with another source, probabley some kind of synth pad.

[KenTheriot]

It has to be the one solo sound source, which is a specialized therapeutic tuning fork. It cannot be mixed with any other sounds. All I need to do is some sort of upward expander (I think) to counteract the level decay of a single strike of a tuning fork.