I am assuming you are intersted in the source of that beep?
There is a Communication Manager feature that will play a tone when a call is conferenced.
See "conference Tone"
I would assume they hear it anytime a call is conferenced.
If not, does it occur if you manyally conference the IVR into the call?
Does it occur if you conference in another station?
Start isolating when the tone occurs and when it doesn't and investigage Communication Manager tones for that scenario. Alternatively the tone may be part of the IVR script.