Yeah but mine is doing it on every bump. You even dont have to go fast or rowdy. The sound is kind of like a Shimano EP801 does.
If it is true, what you say, avinox should say the motor is quiter and not totally silent.
Got a PX 2 weeks ago. At first it was completely silent. After 3000 vertical meters I have now a very annoying rattling sound. Its not far off a shimano ep801 and I am really pissed. As the others said just pull the chain back to hear the noise.