Dopplere can cause autism. you are 100% correct. However, I have 0 hcg in my blood and frequently have to prove to a doctor of my pregnancy.
My midwife actually advised against getting one, one because it can cause unnecessary stress but two the waves and heat it sends out arent good for baby, you definitely shouldn't be doing it very often
Depending what kind of doppler you have, it may be hard to hear but it shouldnt be. Are you using enough gel? Are there headphones attached?