Hello, Welcome to Just Answer and thanks for your question. Dehydration can cause changes in basic metabolic indicators like BUN and also a feeling of palpitations. The BUN wasn't severely elevated and you are fine with increased water intake. If they are quiet and their bodies don't carry too much fat, everybody can see their heart beat in their chest if they are looking for it. You may also be suffering from anxiety which can cause the heart to beat hard and fast. Your primary provider can check and make sure there is nothing else wrong, but your cardiac tests were fine so there probably isn't.
It just means the muscle is working. Often it is from an excess of the neurostimulants (adrenaline) from anxiety. You actually want your heart to beat hard to pump blood through your body, but when you are stressed, scared, or ill, you are conscious of it beating hard more than you are at other times. It also usually beats faster.