I Have A Hollowness In Chest Area,not All The Time

I have a hollowness in chest area,not all the time tho. comes and goes,have had ekg,stress and echograms shows nothing. This all started about 9mo after I had a valve repair and a by pass 3 yrs ago. no pain tho Its annoying Like I need to burp but cannot. I go months nothing then there it is last 5min then gone then back again doesn't bother me to walk or yoga