Hello
Need your help please

My setup:

2 x Beolab 17  +  1 x Beolab 19 SUB

Connection: Wireless with Transmitter 1 and RCA imput Sonos Port

I hear these strange sounds / thunder from the sub every now and then.

When I am not playing anything. See / hear video...

https://youtu.be/2xUQzz05_OY

If anyone knows, please reply 

BW

BinniB