Buzzing sound emanating from my speakers when receiver is set to 'phono'

Make sure there is a ground wire from the turntable to the receiver chassis. 

If you have an external phono preamp, you should definitely run a ground wire from the turntable to the preamp. The phono preamp should already be grounded to the amplifier through the interconnect cables. However, if the hum is still present, try running a second ground wire from the phono preamp chassis to the power amplifier or receiver chassis to possibly eliminate it.

Check your interconnects, too. I have had problems with older interconnects having been over strained and start to have shorts and ground hum.

Jim Hagerman, who makes phono preamps, has this article on eliminating noise in vinyl playback systems:

https://www.hagtech.com/pdf/eliminatingnoise.pdf

My system developed this issue a few weeks ago after moving cables around and the power cable for the amp became close to the phono interconnect, and parallel to it for a few inches.

I rerouted the phono interconnect & amp power cable to fix the problem.  The phono stage is multiplying the signal by a factor of something like 1,000 so it does not take much electromagnetic interference to do nasties.

I once hooked up a turntable with  an unshielded PBJ and got the local AM station!  That was also a sign of a dirty contact acting like a crystal detector. Google can dirty electronic contacts act like an AM detector.

Amusing (off topic) story, In the early '60s I'd been building a crystal set with my much younger brother when we lived in Worcester (UK), about 10 miles from the BBC Droitwich transmission station.  This is a 500kW transmitter at 200kHz.  Somehow the headphone leads were touching the gas stove that we were working next to, in came the BBC Light Program (now BBC 4).  There was even a design for a transistor radio that used the signal from that transmitter as its power source in some radio hobbyist magazine that I read as a young teenager (1950s).

I have posted this root cause analysis protocol in other posts regarding buzzing or humming and distortion.  If you follow the protocol, you should find the issue.  I have further explained reasons.  I have listed then in my opinion of the order of priority   

1. Ground loop:  Are there multiple ground current paths (equipment plugged into different outlets?  Is the turntable and phono preamp grounded properly or has the existing ground wire become faulty? Connect equipment to different outlets, check ground of turntable and phono preamp. 
2. Electromagnetic interference:  Equipment and connections generate EMI that can enter sensitive phono sections.  Have you recently moved equipment or cable paths … or added a new piece of equipment? Relocate equipment or reroute connections.  Faulty interconnect or cable shielding can permit EMI to enter system.  Check all wires for damage.  Have you purchased a new appliance that is on the same line?  Unplug the appliance to see if the problem resolves.  
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