Intermittent electrical noise in system

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

 

 

Glad it worked for you.

As they say here, Happy Listening!

dbtom2, I shall forever be in your debt!!
It WAS a wifi problem.
I disconnected the plug in the wall from the Chromecast and  Voila! The noise went away.
Thanks to you, I can listen to records again.
When I joined this forum yesterday, I never expected to get a solution at all, nevertheless so fast!
Again, a million thank yous!

It's fixed!!!!
Thanks be to dbtom2!!!!!!

Have you tried disconnecting all the digital sources and just running the turntable?

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.

The noise oscillates on and off about two seconds on and four seconds off continually. The sound resembles the voice of a cricket.  It's not loud but it's constantly there intermittently in the background in soft passages.
My components include an old Conrad Johnson pre-amplifier (just checked out by a technician who found no noise,) new Rega Planar 3 turntable, Grado Sonata cartridge,  NuForce dual amps, Theta DAC, Oppo BluRay player used as transport and Dahlqust DQ 20 speakers.
There are no neon signs in the vicinity. I don't know any appliances that cycle on and off like the noise does.  It's maddening!  So much so that I avoid playing records.

and... is anything about 15 years old or more and might electrolytic capacitors in it?

What kind pof noise, hum, high pitched whine, something else.

What components do you have?

Well, if it is intermittent, can you try to isolate it by turning off major appliances, or try to see if something like the fridge is causing this?
I am no electrical engineer, but it sounds like something in your house is periodically running and making this noise. 
It may also be something outside, like a neon sign.(-I think there was a thread on this on Agon). 
So, I guess what I am saying is that it would be best to try to isolate as many variables in order to get some insight as to what may be causing your problem. When you do have some ideas, then we can offer some solutions.
HTH Bob