What is that sound when you cover your ears?

Go into a quiet room and cover your ears and listen to the low, rumbling kinda sound it makes. I think it may be blood flow or something? Maybe you can ear the same sound when you dive your head underwater... But anyway one day I noticed it and I thought it would be a beatiful sound to start one of my songs with and put a slow xylophone over it, which leads to my question: How do i recreate that sound? I've heard people say "use a synthesizer" but I've never really played around with a synthesizer and I don't think it can make the exact sound. Anybody know how i can recreate that exact sound? Thank you!!

Sorry I'm not exactly sure on what category to put this in :/