Our company faced a problem on a 4160 to 600 volt dry type transformer 1000 kva wye/delta solidly grounded on the primary. the problem is big noise which can be heard outside even with all the doors closed in the sub. the noise is like 2 seconds off 4 seconds on, which is way lower than 60 cycle. is not humming noise. Any body who has experience in this matter please answer.