I found it at 8w1d but only faintly and then I couldn't find it for a few days. At 8w4d I was able to start finding it reliably if I have a full bladder. It took a lot of practice though.
I'm thin and have a retroverted ute. Make sure you watch several YouTube videos so you know what you're listening for. Ones that are particularly helpful show you the other sounds and beats you'll hear.