Is it close to the router or wifi antenna? Just move it away

These are most likely interfering magnetic fields (e.g. large loudspeaker nearby) or interfering radio waves (wifi, living room drone, hearing aid 'putt). Using the process of elimination - switching everything off and then on again one by one - you should be able to eliminate the noise. Unless you only watch TV programmes about aquatic life... or test your system with pink Rauschenberg? Good luck!

Personally, it has been working perfectly for many months, with no interfering noise. Review the connections??? If recent changes have been made to the HIFI installation, go back to the previous state and test the desired changes to the installation step by step.

It should remain close to the transmitter then the sound is clear
Transmitter of the Sennheiser