It might be a 'High Leg Delta' problem.
Jim Brown writes:
HIGH LEG DELTA
Figure A-6 shows a variation of the delta configuration that is widely used in North
America, especially in older mixed residential and industrial areas, and in rural areas.
One leg of the delta has a grounded center-tap that serves as the neutral for a singlephase
120/240VAC system, and 208 volts is available for certain industrial applications.
Also known as a Red Leg Delta, or Wild Leg, High Leg Delta systems must have that
phase conductor marked with orange tape, orange finish or similar. This marking is only
required where a connection is made and the neutral conductor is present.
The configuration can work for audio and video systems if there are no other loads on
the transformer. But EMC consultant Neil Muncy has taught us that when High Leg
Delta is used to feed multiple customers from the same transformer, the neutral currents
from one customer can circulate through another customer’s ground system.
When this happens, the neutral feed from the pole-mounted transformer may carry
relatively large neutral currents from those neighboring buildings.
The neutral current will find its way to earth through the system earth at the service entrance,
and in general, the better the earth electrode system, the greater the circulating
current will be! If the path to earth runs near audio equipment or wiring, the magnetic
fields produced by these currents can couple into system wiring, guitar pickups and
dynamic microphones without hum-bucking coils, and even the electronics of audio
gear. The result is hum and buzz that can be eliminated only by eliminating the field.
The hum component is 60 Hz, while the “buzz” consists of harmonics. When called in
to diagnose problems in a small recording studio complex in a renovated industrial
building, Muncy found a High Leg Delta power feed with 7A of neutral current finding
its return path via a water main running under the guitar isolation booth!
The solution is to use a transformer with a single-phase center-tapped secondary (Figure
1), to feed 120/240 v systems in the building, powering it from one of the ungrounded
240 volt phases. What matters is that the shared neutral feed to the building,
with the offending currents, must be eliminated.
http://www.audiosystemsgroup.com/SurgeXPowerGround.pdf