Two different possibilities:
As these are bluetooth enabled, they are susceptible to wireless interference. Which is in accordance with FCC Regulations. It is printed on the back of the Ceres Speaker.
You may be near some radio interference from a nearby power pole, radio station tower, etc that may be pulsing through the environment that your speakers are picking up. Check around your house for anything that may be generating a lot of radio noise. Does this happen when you run your Microwave? or does a neighbor have an amateur radio antenna and amplifier, these spurious signals can create the condition you are mentioning.
The other issue is that there just might be an issue with your speakers in which case we ask that you open a support ticket at https://support.presonus.com
so a support agent can help you get them in for repair.