Hi Kafer Girl, i've had what you're describing for years. Its like a random thumping in one ear and comes and goes. I've been told it it tinitus (not sure if i've spelt that right) and as far as a i know it has nothing to do with pregnancy. I think its caused by loud music damaging your ear drum. I blame mine on too many wild parties back in my uni days My DH gets the same thing every now and then- he used to be a bouncer in a pub so the loud music there probably caused his as well.
Maybe mention it to your doctor next appointment, they'll be able to tell you wether it is tinitus or not.
Hope that helps