Intermittent electrical noise in system

That interval seems like it could be a beacon of some kind. 

If there is a wifi router nearby, turn it off and see if the noise disappears. If it does, see about moving any wifi gear away from your hifi or using ethernet instead of wifi. 

Look for recent electronics you've added to the home. Wireless weather stations can interfere like this. So many products now have wifi capabilities. Bluetooth also. Argh.

If you're in an apartment, good luck.

Glad it worked for you.

As they say here, Happy Listening!