It sounds like it could be her heartbeat. I was in being monitored a few days ago, my HB was 80 & the babies was ranging between 140 & hit 170/180 when I was having contractions.
yeah i was gunna say sounds like hers and not the babys, id try moving it around a bit see if you can pick it up someware else first and if not or if its still 88 call the mw x