Doppler sounds.. More than one.. Twins?!

Ok so messing with my Sonoline B Doppler today. I hear two different sounding heart beats. One @ 148. And the other @156. One was strong and deeper. The other (156) was higher pitch and quicker. I was reading it could be Umbilical cord but I can't find any videos on what the cord sounds like by itself. And info would be great. 

I have had one ultrasound at 7weeks. Said it was just one baby. But I've read a lot of stories on here about mothers not finding out they were having twins till 12+weeks. 

Thanks everyone.