do you also get a little lightheaded during these episodes? if so you're probably experiencing a bit of orthostatic hypotension which is a fancy term that means your blood pressure drops a bit when you stand up. this can happen because gravity pulls blood down into your legs when you stand and it takes the body a moment to get that blood redistributed back up to the brain again. even if you're not experiencing lightheadedness what you're hearing likely is the blood being pumped north after it's been pulled south when you stand up. that whooshing you hear is the heart and baroreceptors working quickly to pump blood upstairs. if this has started recently you might want to consider drinking more water. orthostatic hypotension is by definition an issue of low blood pressure. by increasing your water intake you'll increase the overall volume in your blood vessels and possibly avoid the symptoms you describe. orthostatic hypotension in and of itself usually isn't dangerous. but if you begin fainting or having trouble with balance then it's time to call the doctor because your symptoms could indicate some underlying problem. i hope this helps!
after sitting for 20 minutes and then standing up my hearing goes in and out in rythum with my heartbeat then goes away. this happens regularly after sitting i am 51 5'9" 180lbs muscular build.