Instrument Transformer (CT/PT)

its just a bus PT in a 110kv bus, so a slight loading only always, and no other pt's having this sound,

It's humming because it's happy.

You give no usable information and expect an answer, dream on!

when it's humming, it's ok... when you hear La,LaLaLaLa, La,La,La!

I take it that the humming is the only problem since you have not noted any harmonics or faulty voltage transients. I assume it is with the frequency of the system. Some transformers are just noisy and that could be from faulty manufacture, shipping damage, or handling damage. It is usually a resonance or direct core noise due to looseness or a mechanical stress that is putting pressure on the core. If the PT is withing warranty, call the manufacturer. If it is out of warranty and is providing a clean signal on the secondary, then you will have to decide whether you can live with the sound. If you cannot, consider purchase of a replacement and keep this one for a spare. Repair of a PT is not usually possible and when possible is not usually cost effective. If an outage is in the future and you want to be sure, swap the PT with one from one of the other phases which doesn't make noise. ( I am assuming that it is a single phase PT on a 3 phase system) If it is a 3 phase PT, which is very rare, then the noise could be coming from a harmonic which would be usually 3 or 5 times the frequency of the system. If that is the case then look at the loads. Solid state drives with unbalanced phases are frequent causes. Single phase loads will cause third harmonics. Three phase PT's are usually repairable, but as I said are extremely rare. If you do have a three phase PT, consider a short outage, (minutes) in which the loads on the system are dropped. If the noise stays the same it probably will not mean an outright failure of the PT. If the noise stops, the problem is not in the transformer, but in the load. That is if you are correct and it is a humming sound. If it is a spitting or arcing sound run away from it now and de-energize.

Aside from tightening loose coils and laminate with wedges and by tightening screws...if it can be removed or turned off while drying, I've often cured this problem with 'glyptol', a high solids varnish used for insulation in motors and transformers.