I had this same problem 2 years ago when I had upgraded to Xubuntu 16.04 and whatever version of Audacity was being used then. However, the problem was never resolved. So, I went back to Xubuntu 14.04 and Audacity 2.0.5. Since 18.04 is now out, I decided to try again. This is running Audacity 2.2.1. New versions of everything, but same identical problem.
About every 1 1/2 minutes, a buzzing sounds fades in, lasts for about 10-15 seconds and fades back out. The buzzing sound just about covers the intended recorded sound. This continues forever.
In 14.04, I am recording at 44,100, 32 bit float, medium on both sample rate converters. And it is fine: no problems at all. On 18.04, I lowered it to 22,050, lowest setting on sample rate conversion, tried both 32 float and 16 bit. No good.
I am recording from an Allen & Heath QU32 digital board via a USB cable. 18 channels. Does anyone have any ideas or am I stuck with 14.04 forever?
There are many old posts from almost 2 years ago if they are still available here. They had sound clips of the noise.