they should have been able to tell at that point. if not im sure your next sonogram will be around 20 weeks for an anatomy scan, at which point you will know for sure. i also think if the Dr. thought you were having twins you would have been already considered "at risk" and have had more sonograms scheduled than you do currently. either way keep us posted :)
10 weeks was the last one
they should be able to see not just two heart beats but they should be able to tell you the gender of your child at this point. so depending on when your last sonogram was, twins wouldn't be in question
the whoosh sound is because it we picking up the echo of the heartbeat, the difference in heartbeats be rather the baby was active or sleeping. my son usually runs around 160 but one time he was being sleepy and it was 138. not sure how far along you are but they should know before your too far along rather you have twins :)
I would think that in the 2 months since you had your first ultrasound that they would've been able to determine that by now.
The first ultrasound they thought they saw two and the 3rd one they thought they saw two heart beats
The heart rate changes. It won't be the same every time. The whooshing sound could have been the placenta. They likely would've seen a twin on an ultrasound if there was one.