Many members have discussed pulsatile tinnitus and I have it from time to time where I can hear my heart beat in my ear. I think for me it happens more often when I have sinus congestion that seems to conduct the sound. I actually don't mind because I pay attention to how fast my heart is beating because of allergy and asthma breathing issues. If I have too much congestion in my lungs, my heart rate goes up which is sometimes a clue that there is too much trapped phlegm that can become an infection, and I can take action to clear it.
Your blood pressure will change and adjust as you change your body position, and perhaps that may be why you notice a change in the loudness of the thumping. Perhaps having your hands in hot water also affects blood pressure or heart rate. If you ask your doctors about how blood pressure or heart rate affects pulsatile tinnitus, you may get an answer.